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Share-Based Compensation Plans - Restricted Stock Activity (Details) - Restricted stock - USD ($)
$ / shares in Units, shares in Thousands, $ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Shares        
Unvested at beginning of period (in shares) 451 405 463  
Granted (in shares) 167 232 179  
Vested (in shares) (133) (172) (217)  
Forfeited (in shares) (37) (14) (20)  
Unvested at end of period (in shares) 448 451 405 463
Wtd. Avg. Price Per Share        
Unvested at beginning of period (in dollars per share) $ 70 $ 64 $ 57  
Granted (in dollars per share) 98 75 76  
Vested (in dollars per share) 79 62 59  
Forfeited (in dollars per share) 80 76 60  
Unvested at end of period (in dollars per share) $ 77 $ 70 $ 64 $ 57
Equity Other Than Options, Additional [Abstract]        
Unvested (in years) 1 year 2 months 12 days 1 year 4 months 24 days 1 year 3 months 18 days 1 year 2 months 12 days
Unvested $ 56