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Equity Incentive Plans - Changes in performance stock awards (Details) - Performance stock awards - USD ($)
$ / shares in Units, shares in Thousands, $ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Stock units activity      
Outstanding at the beginning of the period (in shares) 1,248    
Granted (in shares) 415    
Adjustment for performance achievement (in shares) (267)    
Vested (in shares) (702)    
Forfeited (in shares) (47)    
Outstanding at the end of the period (in shares) 1,181 1,248  
Weighted average grant date fair value      
Weighted average grant date fair value, nonvested at the beginning of the period (in dollars per share) $ 77.35    
Weighted average grant date fair value, granted (in dollars per share) 92.49 $ 92.88 $ 78.47
Weighted average grant date fair value, adjustment for performance achievement (in dollars per share) 62.32    
Weighted average grant date fair value, vested (in dollars per share) 62.32    
Weighted average grant date fair value, forfeited (in dollars per share) 87.83    
Weighted average grant date fair value, nonvested at the end of the period (in dollars per share) $ 87.78 $ 77.35  
Total fair value of restricted stock and restricted stock units vested $ 65 $ 15 $ 17