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Shareholders' Equity - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions | 1 Months Ended | 3 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Dec. 31, 2017 | Mar. 31, 2018 | Mar. 31, 2017 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Jan. 01, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Rate of options exercisable per year, commencing one year after grant |  |  |  | 20% per year commencing one year after grant |  |  |
Duration of options expiration, after date of grant |  |  |  | 10 years |  |  |
Total intrinsic value of options exercised |  |  |  | $ 57 | $ 65 | $ 38 |
Cash received from the exercise of stock options |  |  |  | 29 | 34 | 32 |
Tax benefit related to exercise of stock options |  |  |  | 9 | 18 | 11 |
Total compensation expense related to stock incentive plans |  |  |  | 23 | 30 | 28 |
Tax benefit related to compensation costs |  |  |  | 13 | 27 | 19 |
Net of tax unrealized gains (losses) related to credit-only impaired securities | $ 68 |  |  | $ 58 | 68 | $ 52 |
Impact of the U.S. corporate tax rate change on AOCI | 4 |  |  |  | $ 4 |  |
Minimum |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Restricted Common Stock vesting period |  |  |  | 3 years |  |  |
Maximum |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Restricted Common Stock vesting period |  |  |  | 4 years |  |  |
Stock Incentive Plan |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Common Stock reserved for issuance under stock incentive plans (shares) |  |  |  | 7,000,000.0 |  |  |
Common Stock options issued under stock incentive plan (shares) |  |  |  | 0 | 0 | 0 |
Annual Equity Bonus Plan |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Other benefit plans, shares issued (shares) |  | 45,804 | 47,826 |  |  |  |
Common Stock issued, fair value per share (USD per share) |  | $ 115.49 | $ 96.13 |  |  |  |
Stock Options |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Unrecognized compensation expense related to equity-based awards that have yet to vest |  |  |  | $ 3 |  |  |
Weighted average period of cost expected to be recognized |  |  |  | 1 year |  |  |
Restricted Stock |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Restricted Common Stock vested during period, fair value |  |  |  | $ 10 | $ 14 | $ 10 |
Unrecognized compensation expense related to equity-based awards that have yet to vest |  |  |  | $ 35 |  |  |
Weighted average period of cost expected to be recognized |  |  |  | 2 years 6 months |  |  |
Accounting Standards Update 2018-02 | Tax Cuts and Jobs Act of 2017 |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Tax Cuts and Jobs Act of 2017, Reclassification from AOCI to Retained Earnings, Tax Effect | $ 145 |  |  |  |  |  |
Equity securities | Accounting Standards Update 2016-01 |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Impact of the U.S. corporate tax rate change on AOCI |  |  |  |  |  |  | $ (221)

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