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Income Taxes - Reconciliation of Change in Unrecognized Tax Benefits (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Gross Unrecognized Tax Benefits |  |  |
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward] |  |  |
Beginning balance | $ 431 | $ 440 | $ 86
Additions for tax positions related to the current year | 33 | 23 | 28
Additions for tax positions related to prior years | 3 | 12 | 402
Reductions for tax positions related to prior years due to IRS and other settlements | (16) | (44) | (76)
Ending balance | 451 | 431 | 440
Portion of balance at December 31, that, if recognized, would impact the effective income tax rate | 153 |  |
Accrued Interest and Penalties |  |  |
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward] |  |  |
Beginning balance | 31 | 35 | 29
Additions for tax positions related to the current year | 0 | 17 | 0
Additions for tax positions related to prior years | 21 | 4 | 25
Reductions for tax positions related to prior years due to IRS and other settlements | (6) | (25) | (19)
Ending balance | 46 | 31 | 35
Portion of balance at December 31, that, if recognized, would impact the effective income tax rate | 35 |  |
Gross Tax, Interest and Penalties |  |  |
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward] |  |  |
Beginning balance | 462 | 475 | 115
Additions for tax positions related to the current year | 33 | 40 | 28
Additions for tax positions related to prior years | 24 | 16 | 427
Reductions for tax positions related to prior years due to IRS and other settlements | (22) | (69) | (95)
Ending balance | 497 | $ 462 | $ 475
Portion of balance at December 31, that, if recognized, would impact the effective income tax rate | $ 188 |  |

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- Definition A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period. + References No definition available. + Details
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- Definition Amount of unrecognized tax benefits. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 45 -Paragraph 10B -URI http://asc.fasb.org/extlink&oid=120406818&loc=SL37586934-109318 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=84230637&loc=SL6600010-109319 + Details
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- Definition Amount of decrease in unrecognized tax benefits resulting from settlements with taxing authorities. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=84230637&loc=SL6600010-109319 + Details
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- Definition Amount of increase in unrecognized tax benefits resulting from tax positions that have been or will be taken in current period tax return. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=84230637&loc=SL6600010-109319 + Details
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- Definition Amount of increase in unrecognized tax benefits resulting from tax positions taken in prior period tax returns. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=84230637&loc=SL6600010-109319 + Details
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