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Shareholders' Equity - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions | 3 Months Ended | 9 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Sep. 30, 2018 | Mar. 31, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017 | Jun. 30, 2018 | Jan. 01, 2018 | Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Net of tax unrealized gains (losses) related to credit-only impaired securities | $ 64 |  |  | $ 64 |  | $ 67 |  | $ 68
Cumulative effect of accounting change |  |  |  |  |  |  |  | $ 4
Compensation expense related to stock incentive plans | $ 6 |  | $ 7 | $ 17 | $ 24 |  |  |
Stock Incentive Plan |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Restricted Common Stock, shares issued (shares) |  |  |  | 200,625 |  |  |  |
Restricted Common Stock, fair value per share (USD per share) |  |  |  | $ 112.86 |  |  |  |
Common Stock options issued under stock incentive plan (shares) |  |  |  | 0 |  |  |  |
Annual Equity Bonus Plan |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Other benefit plans, shares issued (shares) |  | 45,804 |  |  |  |  |  |
Common Stock issued, fair value per share (USD per share) |  | $ 115.49 |  |  |  |  |  |
Accounting Standards Update 2016-01 | Equity securities |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Cumulative effect of accounting change |  |  |  |  |  |  | $ (221) |

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