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Shareholders' Equity - Narrative (Details) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
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 $ 61   $ 58    
Cumulative effect of accounting change         $ 4
Compensation expense related to stock incentive plans $ 6 $ 5      
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)