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Accounting Policies - Narrative (Details) - USD ($) shares in Millions, $ in Millions | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Jan. 01, 2016 | Dec. 31, 2014
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Net unrealized gain on equity securities included in AOCI |  | $ 840 | $ 404 |  |  |
Impact of the U.S. corporate tax rate change on AOCI |  | 0 |  |  |  | $ 2
Tax benefit related to exercise of stock options |  | $ 18 | $ 11 | $ 16 |  |
Weighted average common shares adjustment related to stock-based compensation |  | 2.0 | 1.6 | 1.8 |  |
Anti-dilutive potential common shares related to stock-based compensation plans |  | 0.0 | 0.4 | 1.1 |  |
Maturities of short term investments |  | 3 months |  |  |  |
Accounting Standards Update 2014-09 |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Expected effect on consolidated revenues under new revenue recognition guidance effective in 2018 |  | 2.00% |  |  |  |
Retained Earnings |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Impact of the U.S. corporate tax rate change on AOCI |  | $ (145) |  |  |  | $ 2
Retained Earnings | Maximum |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Impact of the U.S. corporate tax rate change on AOCI |  |  |  |  | $ 1 |
Stock Options | Capital surplus under previous accounting guidance |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Tax benefit related to exercise of stock options |  | 17 | $ 9 |  |  |
Equity securities |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Net unrealized gain on equity securities included in AOCI | [1] | 221 | $ 98 |  |  |
Equity securities | Accounting Standards Update 2016-01 |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Net unrealized gain on equity securities included in AOCI |  | 221 |  |  |  |
Tax Cuts and Jobs Act of 2017 | Accounting Standards Update 2018-02 |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Impact of the U.S. corporate tax rate change on AOCI |  | $ 145 |  |  |  |
[1] | As discussed in Note A — “Accounting Policies — Investments,” effective January 1, 2018, all equity securities currently classified as “available for sale” will be required to be carried at fair value through net earnings instead of AOCI.

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