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Stock Compensation - Restricted Stock Units Narrative (Details) - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
6 Months Ended 12 Months Ended
Dec. 31, 2016
Dec. 31, 2018
Dec. 31, 2017
Restricted Stock Units (RSUs)      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award requisite service period   3 years  
Shares vested and issued each year employee remains in service   33.00%  
Total unrecognized compensation cost related to award, weighted average period   16 months  
Restricted stock units, unvested (in shares)   1.1 0.7
Equity instrument, granted (in shares)   0.7  
Vested or forfeited (in shares)   0.3  
Total compensation cost for award recognized in income   $ 27 $ 25
Total compensation cost for award recognized in income, tax benefit   7 $ 10
Total unrecognized compensation cost related to award   $ 13  
Parent Company | Restricted Stock Units (RSUs)      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Equity instrument granted in period, weighted average grant date fair value (in dollars per share) $ 53.83 $ 43.81 $ 49.25
Equity instrument, granted (in shares) 0.7    
One-year vesting period      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Total unrecognized compensation cost related to award, weighted average period   1 year  
Two-year vesting period      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Total unrecognized compensation cost related to award, weighted average period   2 years  
Three-year vesting period      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Total unrecognized compensation cost related to award, weighted average period   3 years