NoTE 12: FAIr vAluE MEASurEMENT NoTE 13: INCoME pEr CoMMoN ShArE F-28 [ HL:10-K ]  A total of 157,814 shares of preferred stock were outstanding at December 31, 2012. Potential dilutive common shares include outstanding stock options, restricted stock awards, stock units, warrants and convertible preferred stock  for periods in which we have reported net income. For periods in which we reported net losses, potential dilutive common shares are excluded, as  their conversion and exercise would not reduce earnings per share. Under the if-converted method, preferred shares would not dilute earnings per  share in any of the periods presented. We calculate basic earnings per share using, as the denominator, the weighted average number of common shares outstanding during the period.  Diluted earnings per share uses, as its denominator, the weighted average number of common shares outstanding during the period plus the effect of  potential dilutive common shares during the period using the treasury stock method for options, warrants, and restricted stock units, and if-converted  method for convertible preferred shares. During the second quarter of 2010, we began utilizing financially-settled forward contracts to manage the exposure of changes in prices of zinc and  lead contained in our concentrate shipments that have not reached final settlement. We also began utilizing financially-settled forward contracts in the  second quarter of 2010 to manage the exposure of changes in prices of zinc and lead contained in our forecasted future concentrate shipments (see  Note 10 for more information). These contracts do not qualify for hedge accounting, and are marked-to-market through earnings each period. The fair  value of each contract represents the difference between the forward metal price for the contract settlement period as of the measurement date and  the contract settlement metal price, multiplied by the quantity of metal involved in the contract. Trade accounts receivable consist of amounts due to us for shipments of concentrates and doré sold to smelters and refiners. Revenues and the  corresponding accounts receivable for sales of metals products are recorded when title and risk of loss transfer to the customer (generally at the time  of shipment). Sales of concentrates are recorded using estimated forward prices for the anticipated month of settlement applied to our estimate of  payable metal quantities contained in each shipment. Sales are recorded net of estimated treatment and refining charges, which are also impacted by  changes in metals prices and quantities of contained metals. We must estimate the prices at which sales of our concentrates will be settled due to the  time elapsed between shipment and final settlement with the smelter. Receivables for previously recorded concentrate sales are adjusted to reflect  estimated settlement metals prices at the end of each period until final settlement by the smelter. We obtain the forward metals prices used each  period from a pricing service. Changes in metal prices between shipment and final settlement will result in changes to revenues previously recorded  upon shipment. The embedded derivative contained in our concentrate sales is adjusted to fair market value through earnings each period prior to  final settlement. Our current and non-current investments consist of marketable equity securities which are valued using quoted market prices for each security  multiplied by the number shares held by us. Cash and cash equivalents consist primarily of money market funds and are valued at cost, which approximates fair value. Current and non-current restricted cash balances consist primarily of certificates of deposit and U.S. Treasury securities and are valued at cost, which  approximates fair value. The table below sets forth our assets and liabilities (in thousands) that were accounted for at fair value on a recurring basis and the fair value  calculation input hierarchy level that we have determined applies to each asset and liability category. See  Note 8  for information on the fair values of  our defined benefit pension plan assets.  Balance at   Balance at  Input December 31, December 31, Hierarchy 2012 2011 Level Assets:  Cash and cash equivalents:  Money market funds and other bank deposits  $  190,984  $  266,463  Level 1 Available for sale securities:  Equity securities – mining industry  9,614 3,923 Level 1 Trade accounts receivable:  Receivables from provisional concentrate sales  17,555 10,996 Level 2 Derivative contracts: Base metal forward contracts  5,606 32,750 Level 2 Restricted cash balances:  Certificates of deposit and other bank deposits 871  866  Level 1 Total assets $ 224,630  $ 314,998 Liabilities:  Derivative contracts:  Base metal forward contracts  $ 2,483 $ — Level 2