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Shareholders' Equity - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions | 3 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Mar. 31, 2017 | Mar. 31, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
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 |  |  | $ 65 | $ 38 | $ 52 |
Cash received from the exercise of stock options |  |  | 33 | 32 | 47 |
Tax benefit related to exercise of stock options |  |  | 18 | 11 | 16 |
Total compensation expense related to stock incentive plans |  |  | 30 | 28 | 27 |
Tax benefit related to compensation costs |  |  | 27 | 19 | 8 |
Net of tax unrealized gains (losses) related to credit-only impaired securities |  |  | 68 | $ 52 | 51 |
Impact of the U.S. corporate tax rate change on AOCI |  |  | $ 0 |  |  | $ 2
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) |  |  | 8,000,000.0 |  |  |
Common Stock options issued under stock incentive plan (shares) |  |  | 0 | 0 |  |
Annual Equity Bonus Plan |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Other benefit plans, shares issued (shares) | 47,826 | 40,336 |  |  |  |
Common Stock issued, fair value per share (USD per share) | $ 96.13 | $ 71.05 |  |  |  |
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 |  |  | $ 7 |  |  |
Weighted average period of cost expected to be recognized |  |  | 1 year 8 months 12 days |  |  |
Restricted Stock |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Unrecognized compensation expense related to equity-based awards that have yet to vest |  |  | $ 31 |  |  |
Weighted average period of cost expected to be recognized |  |  | 2 years 7 months 6 days |  |  |
Capital surplus |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Tax benefit related to exercise of stock options |  |  |  |  | $ 12 |
Capital surplus under previous accounting guidance | Stock Options |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Tax benefit related to exercise of stock options |  |  | $ 17 | $ 9 |  |
Accounting Standards Update 2018-02 | Tax Cuts and Jobs Act of 2017 |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Impact of the U.S. corporate tax rate change on AOCI |  |  | $ 145 |  |  |

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