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Employee Stock-based Compensation Plans Weighted-Average Fair Value and Total Fair Value of Shares Vested (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Weighted-average fair value of restricted shares units granted (in usd per share) $ 34.88    
Restricted Stock Units      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Weighted-average fair value of restricted shares units granted (in usd per share) $ 34.88 $ 26.61 $ 32.82
Total fair value of shares vested (in millions) $ 50 $ 61 $ 45