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Accounting Policies - Narrative (Details) - USD ($) $ in Millions |  | 1 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | ---
Dec. 31, 2017 | Dec. 31, 2017 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Jan. 01, 2019 | Jan. 01, 2018
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Cumulative effect of accounting change |  |  |  |  |  |  | $ 4
Operating lease liability |  |  | $ 180 |  |  | $ 198 |
Operating lease right-of-use asset |  |  | 158 |  |  | 174 |
Deferred rent and lease incentives |  |  | $ 22 |  |  | $ 24 |
Tax Cuts and Jobs Act, reclassification from AOCI to retained earnings, tax effect |  |  |  |  | $ 0 |  |
Weighted average common shares adjustment related to stock-based compensation |  |  | 1,100,000 | 1,600,000 | 2,000,000.0 |  |
Anti-dilutive potential common shares related to stock-based compensation plans |  |  | 0 | 0 | 0 |  |
Maturities of short term investments |  |  | 3 months |  |  |  |
Equity securities |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Available-for-sale securities, Equity securities | $ 1,600 | $ 1,600 |  |  | $ 1,600 |  |
Equity securities | Accounting Standards Update 2016-01 |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Cumulative effect of accounting change |  |  |  |  |  |  | (221)
Tax Cuts and Jobs Act of 2017 | Accounting Standards Update 2018-02 |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Tax Cuts and Jobs Act, reclassification from AOCI to retained earnings, tax effect | $ 145 | $ 145 |  |  |  |  |
Limited partnerships and similar investments | Accounting Standards Update 2016-01 |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Cumulative effect of accounting change |  |  |  |  |  |  | $ (4)
Equity securities still owned | Equity securities | Accounting Standards Update 2016-01 |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Net holding gains (losses) on equity securities |  |  | $ 207 | $ (257) |  |  |

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