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Accounting Policies - Narrative (Details) - USD ($) $ in Millions | 12 Months Ended |  |  |
--- | --- | --- | --- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Jan. 01, 2019 | Jan. 01, 2018 | Dec. 31, 2017
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Cumulative effect of accounting change | $ (6,789) | $ (6,269) | $ (4,972) |  |  | $ (5,331)
Operating lease liability | 159 | 180 |  | $ 198 |  |
Operating lease right-of-use asset | 139 | 158 |  | 174 |  |
Deferred rent and lease incentives | $ 20 | $ 22 |  | $ 24 |  |
Weighted average common shares adjustment related to stock-based compensation | 500,000 | 1,100,000 | 1,600,000 |  |  |
Anti-dilutive potential common shares related to stock-based compensation plans | 0 | 0 | 0 |  |  |
Maturities of short term investments | 3 months |  |  |  |  |
AOCI attributable to parent |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Cumulative effect of accounting change | $ (1,273) | $ (863) | $ (48) |  |  | (813)
Retained Earnings |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Cumulative effect of accounting change | $ (4,149) | (4,009) | $ (3,588) |  |  | (3,248)
Cumulative effect, period of adoption, adjustment |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Cumulative effect of accounting change |  |  |  |  |  | (4)
Cumulative effect, period of adoption, adjustment | AOCI attributable to parent |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Cumulative effect of accounting change |  | 0 |  |  | $ 221 | 221
Cumulative effect, period of adoption, adjustment | Retained Earnings |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Cumulative effect of accounting change |  | (7) |  |  |  | (225)
Accounting Standards Update 2016-01 | Cumulative effect, period of adoption, adjustment | AOCI attributable to parent |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Cumulative effect of accounting change |  |  |  |  |  | 221
Accounting Standards Update 2016-13 | Cumulative effect, period of adoption, adjustment | Retained Earnings |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Cumulative effect of accounting change |  | $ (7) |  |  |  |
Equity securities |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Available-for-sale securities, Equity securities |  |  |  |  |  | 1,600
Limited partnerships and similar investments | Accounting Standards Update 2016-01 | Cumulative effect, period of adoption, adjustment |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Cumulative effect of accounting change |  |  |  |  |  | $ (4)

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