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Compensation Plans - Restricted Stock Awards Narrative (Details) - USD ($)
shares in Thousands, $ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Restricted Shares      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock-based awards, vesting period 3 years    
Restricted Shares | Minimum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock-based awards, vesting period 3 years    
Percentage of performance metrics during performance period 0.00%    
Restricted Shares | Maximum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock-based awards, vesting period 6 years    
Percentage of performance metrics during performance period 200.00%    
Restricted Stock and Performance Restricted Stock Units      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares withheld to offset tax withholding obligations (in shares) 141 157 237
Weighted-average remaining vesting period of restricted stock and restricted stock units 2 years    
Total fair values (at vesting) of restricted stock and restricted stock units vested $ 10 $ 15 $ 24