67 AGL Energy Limited Annual Report 2011 (l) Trade and other receivables Loans and receivables Impairment of fi nancial assets impaired, the cumulative gain or loss previously accumulated in the  investments revaluation reserve is reclassified to profit or loss. Trade and other receivables are recognised at amortised cost, less  an allowance for doubtful debts. Trade receivables are generally  due for settlement within 30 days. Collectibility of trade receivables is reviewed on an ongoing basis.  An allowance for doubtful debts is recognised when there is  objective evidence that the consolidated entity will not be able to  collect all amounts due. Financial difficulties of the debtor, default  payments or debts overdue are considered objective evidence of  impairment. The amount of the allowance is the difference  between the asset’s carrying amount and the present value of  estimated future cash flows. Loans and receivables are non-derivative financial assets with fixed  or determinable payments that are not quoted in an active market.  Loans and receivables are measured at amortised cost using the  effective interest method, less any impairment. They are included  in current assets, except for those with maturities greater than  12 months after the end of the reporting period which are classified  as non-current assets. Loans and receivables are included in trade  and other receivables and other financial assets in the statement of  fi nancial position. The amount of the impairment loss is recognised in the income  statement within other expenses. When a trade receivable is  uncollectible, it is written off against the allowance account.  Subsequent recoveries of amounts previously written off are  credited against other expenses in the income statement. Financial assets, other than those at fair value through profit or  loss, are assessed for indicators of impairment at the end of each  reporting period. Financial assets are considered to be impaired  where there is objective evidence that, as a result of one or more  events that occurred after the initial recognition of the financial  asset, the estimated future cash flows of the asset have been  effected. Unbilled revenue represents estimated gas and electricity services  supplied to customers but unbilled at the end of the reporting period. For financial assets measured at amortised cost, the amount of  the impairment loss recognised in profit or loss is the difference  between the asset’s carrying amount and the present value of  estimated future cash flows, discounted at the financial asset’s  original effective interest rate. Impairment of trade receivables is  recognised in accordance with the accounting policy set out in  Note 1(l). When an available-for-sale financial asset is considered  to be impaired, cumulative gains or losses previously recognised in  other comprehensive income are reclassified to profit or loss. Stocks and materials are valued at the lower of cost and estimated  net realisable value. Costs, including an appropriate portion of fixed  and variable overhead expenses, are assigned to inventory on hand  by the method most appropriate to each particular class of  inventory, with the majority being valued on a first-in-first-out  basis. Finished goods includes gas inventory. All financial assets are recognised and derecognised on trade date  where the purchase or sale of a financial asset is under a contract  whose terms require delivery of the financial asset within the  timeframe established by the market concerned, and are initially  measured at fair value, plus transaction costs, except for those  financial assets classified as at fair value through profit or loss,  which are initially measured at fair value. With the exception of available-for-sale equity instruments, if, in a  subsequent period, the amount of the impairment loss decreases  and the decrease can be related objectively to an event occurring  after the impairment was recognised, the previously recognised  impairment loss is reversed through profit or loss. (m) Inventories (n) Financial assets In respect of available-for-sale equity instruments, impairment  losses previously recognised in profit or loss are not reversed  through profit or loss. Any increase in fair value subsequent to an  impairment loss is recognised in other comprehensive income. Financial assets are classified into the following specified categories:  financial assets ‘at fair value through profit or loss’,  ‘available-for-sale’ financial assets, and ‘loans and receivables’. The  classification depends on the nature and purpose of the financial  assets and is determined at the time of initial recognition. Financial assets are derecognised when the rights to receive cash  flows from the financial assets has expired or when the  consolidated entity transfers substantially all the risks and rewards  of the financial assets. If the consolidated entity neither retains nor  transfers substantially all of the risks and rewards of ownership, it  derecognises the asset if it has transferred control of the assets. Financial assets at fair value through profit or loss Financial assets classified as held for trading are included in the  category ‘financial assets at fair value through profit or loss’.  Financial assets are classified as held for trading if acquired  principally for the purpose of selling in the short-term. Derivatives  are also classified as held for trading unless they are designated as  effective hedging instruments. Gains or losses on financial assets  held for trading are recognised in profit or loss and the related  assets are classified as current assets in the statement of financial  position. Fair value is determined in the manner described in  Note 52. Non-current assets and disposal groups are classified as held for  sale if their carrying amount will be recovered principally through a  sale transaction rather than through continuing use. This condition  is regarded as met only when the sale is highly probable and the  non-current asset or disposal group is available for immediate sale  in its present condition. Management must be committed to the  sale, which should be expected to qualify for recognition as a  completed sale within one year from the date of classification. Derecognition of fi nancial assets (o) Non-current assets and disposal groups held for sale Available-for-sale fi nancial assets Non-current assets (including those that are part of a disposal  group) are not depreciated or amortised while they are classified as  held for sale. Non-current assets and disposal groups classified as  held for sale are measured at the lower of their previous carrying  amount and fair value less costs to sell. Listed shares held by the consolidated entity that are traded in an  active market are classified as available-for-sale and are stated at  fair value. Fair value is determined in the manner described in  Note 52. Gains and losses arising from changes in fair value are  recognised directly in other comprehensive income and  accumulated in the investments revaluation reserve, with the  exception of impairment losses, which are recognised in profit or  loss. Where the investment is disposed of or is determined to be 