Tax Uncertainties.  We participate in the Internal Revenue Service (IRS) Compliance Assurance Process (CAP), a real- time audit of our consolidated federal corporate income tax return. The IRS has examined our consolidated federal income tax returns through 2019. For all periods open to examination by tax authorities, we periodically assess our liabilities and contingencies based on the latest available information. Where we believe there is more than a 50% chance that our tax position will not be sustained, we record our best estimate of the resulting tax liability, including interest, in the Consolidated Financial Statements. We include any interest or penalties incurred in connection with income taxes as part of income tax expense. Based on all known facts and circumstances and current tax law, we believe the total amount of any unrecognized tax benefits on December 31, 2020, was not material to our results of operations, financial condition or cash flows. In addition, there are no tax positions for which it is reasonably possible that the unrecognized tax benefits will vary significantly over the next 12 months, producing, individually or in the aggregate, a material effect on our results of operations, financial condition or cash flows. Accounts receivable represent amounts billed and currently due from customers. Payment is typically received from our customers either at periodic intervals (e.g., biweekly or monthly) or upon achievement of contractual milestones. Accounts receivable consisted of the following: Receivables from non-U.S. government customers included amounts related to long-term production programs for the Spanish Ministry of Defence of $1.6 billion and $1.7 billion on December 31, 2020 and 2019, respectively. A different ministry, the Spanish Ministry of Industry, has funded work on these programs in advance of costs incurred by the company. The cash advances are reported on the Consolidated Balance Sheet in current customer advances and deposits and will be repaid to the Ministry of Industry as we collect on the outstanding receivables from the Ministry of Defence. The net amounts for these programs on December 31, 2020 and 2019, were advance payments of $245 and $295, respectively. With respect to our other receivables, we expect to collect substantially all of the year-end 2020 balance during 2021. Unbilled receivables represent revenue recognized on long-term contracts (contract costs and estimated profits) less associated advances and progress billings. These amounts will be billed in accordance with the agreed-upon contractual terms. Unbilled receivables consisted of the following: 73 December 31 2020 2019 Unbilled revenue $ 36,657  $ 33,481  Advances and progress billings (28,633) (25,624) Net unbilled receivables $ 8,024  $ 7,857  December 31 2020 2019 Non-U.S. government $ 1,701  $ 1,847  U.S. government 1,040  1,076  Commercial 420  621  Total accounts receivable $ 3,161  $ 3,544  G. ACCOUNTS RECEIVABLE H. UNBILLED RECEIVABLES