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Accounting Policies - Narrative (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | ---
Jun. 30, 2021 | Jun. 30, 2020 | Jun. 30, 2021 | Jun. 30, 2020 | Mar. 31, 2021 | Dec. 31, 2020 | Mar. 31, 2020 | Dec. 31, 2019
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Cumulative effect of accounting change | $ 5,601 | $ 6,126 | $ 5,601 | $ 6,126 | $ 6,685 | $ 6,789 | $ 5,047 | $ 6,269
Operating lease liability | $ 143 |  | $ 143 |  |  | $ 159 |  |
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List] | us-gaap:OtherLiabilities |  | us-gaap:OtherLiabilities |  |  | us-gaap:OtherLiabilities |  |
Operating lease right-of-use asset | $ 124 |  | $ 124 |  |  | $ 139 |  |
Weighted average common shares adjustment related to stock-based compensation (shares) | 600,000 | 300,000 | 600,000 | 0 |  |  |  |
Anti-dilutive potential common shares related to stock-based compensation plans (shares) | 0 | 0 | 0 | 600,000 |  |  |  |
Maturities of short term investments |  |  | 3 months |  |  |  |  |
Retained Earnings |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Cumulative effect of accounting change | $ 4,023 | $ 3,685 | $ 4,023 | $ 3,685 | $ 4,354 | $ 4,149 | $ 3,616 | 4,009
Cumulative effect adjustment in period of adoption | Retained Earnings |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Cumulative effect of accounting change |  |  |  |  |  |  |  | 7
Cumulative effect adjustment in period of adoption | Retained Earnings | Accounting Standards Update 2016-13 |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Cumulative effect of accounting change |  |  |  |  |  |  |  | $ 7

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