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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 | Dec. 31, 2017 | Mar. 31, 2018 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Jan. 01, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Restricted Common Stock vesting period |  |  |  | four-year |  |  |
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 |  |  |  | $ 46 | $ 57 | $ 65 |
Cash received from the exercise of stock options |  |  |  | 31 | 29 | 34 |
Tax benefit related to exercise of stock options |  |  |  | 8 | 9 | 18 |
Total compensation expense related to stock incentive plans |  |  |  | 23 | 23 | 30 |
Tax benefit related to compensation costs |  |  |  | 13 | 13 | 27 |
Net of tax unrealized gains (losses) related to credit-only impaired securities | $ 68 | $ 68 |  | $ 55 | $ 58 | 68 |
Cumulative effect of accounting change |  |  |  |  |  |  | $ 4
Tax Cuts and Jobs Act, reclassification from AOCI to retained earnings, tax effect |  |  |  |  |  | $ 0 |
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, reclassification from AOCI to retained earnings, tax effect | $ 145 | $ 145 |  |  |  |  |
Equity securities | Accounting Standards Update 2016-01 |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Cumulative effect of accounting change |  |  |  |  |  |  | $ (221)
Stock Incentive Plan |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Net of Forfeitures |  |  |  | 0 | 0 | 0 |
Common Stock reserved for issuance under stock incentive plans (shares) |  |  |  | 4,500,000 |  |  |
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 |  |  |  |
Restricted Stock |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Restricted Common Stock vested during period, fair value |  |  |  | $ 11 | $ 10 | $ 14 |
Unrecognized compensation expense related to equity-based awards that have yet to vest |  |  |  | $ 36 |  |  |
Weighted average period of cost expected to be recognized |  |  |  | 2 years 4 months 24 days |  |  |
Maximum | 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 |  |  |  | $ 1 |  |  |

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