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Equity Incentive Plans - Changes in performance stock awards (Details) - USD ($)
$ / shares in Units, shares in Thousands, $ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Weighted average grant date fair value      
Tax (expense) benefit realized related to all stock-based compensation included in shareholder's equity     $ 30
Performance stock awards      
Stock units activity      
Outstanding at the beginning of the period (in shares) 1,090    
Granted (in shares) 402    
Vested (in shares) (161)    
Adjustment for performance achievement (in shares) (34)    
Forfeited (in shares) (49)    
Outstanding at the end of the period (in shares) 1,248 1,090  
Weighted average grant date fair value      
Weighted average grant date fair value, nonvested at the beginning of the period (in dollars per share) $ 70.35    
Weighted average grant date fair value, granted (in dollars per share) 92.88 $ 78.47 $ 62.32
Weighted average grant date fair value, adjustment for performance achievement (in dollars per share) 70.53    
Weighted average grant date fair value, vested (in dollars per share) 70.53    
Weighted average grant date fair value, forfeited (in dollars per share) 76.33    
Weighted average grant date fair value, nonvested at the end of the period (in dollars per share) $ 77.35 $ 70.35  
Total fair value of restricted stock and restricted stock units vested $ 15 $ 17 $ 28
Stock options      
Weighted average grant date fair value      
Tax (expense) benefit realized related to all stock-based compensation included in shareholder's equity     $ 23