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Accounting Policies - Narrative (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended |  |
--- | --- | --- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017 | Jan. 01, 2018 | Dec. 31, 2017
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Net unrealized gain on equity securities included in AOCI |  | $ 93 |  | $ 93 |  |  | $ 840
Cumulative effect of accounting change |  |  |  |  |  |  | 4
Weighted average common shares adjustment related to stock-based compensation (shares) |  | 1,600,000 | 1,900,000 | 1,700,000 | 2,000,000 |  |
Anti-dilutive potential common shares related to stock-based compensation plans (shares) |  | 0 | 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
Net unrealized gain on equity securities included in AOCI | [1] |  |  |  |  |  | 221
Equity securities, trading at fair value |  |  |  |  |  |  | 62
Equity securities | Accounting Standards Update 2016-01 |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Net unrealized gain on equity securities included in AOCI |  |  |  |  |  |  | $ 221
Cumulative effect of accounting change |  |  |  |  |  | $ (221) |
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 |  | $ 27 |  | $ (35) |  |  |
[1] | As discussed in Note A — “Accounting Policies — Investments,” effective January 1, 2018, all equity securities other than those accounted for under the equity method are carried at fair value through net earnings.

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