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Shareholders' Equity - Narrative (Details) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Mar. 31, 2019
Dec. 31, 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 $ 59   $ 59   $ 61 $ 58    
Cumulative effect of accounting change               $ 4
Compensation expense related to stock incentive plans $ 6 $ 6 $ 12 $ 11        
Stock Incentive Plan                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Restricted Common Stock, shares issued (shares)     232,565          
Restricted Common Stock, fair value per share (USD per share)     $ 99.28          
Common Stock options issued under stock incentive plan (shares)     0          
Accounting Standards Update 2016-01 | Equity securities                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Cumulative effect of accounting change             $ (221)