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Equity - Restricted stock units (Details) - USD ($)
$ / shares in Units, shares in Thousands, $ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Other disclosures        
Share-based payments subject to redemption $ 37 $ 36 $ 30 $ 24
Restricted Stock Units (RSUs)        
Share-based compensation        
Vesting terms 3 years      
Service period over which compensation expense would be amortized 1 year      
Restricted stock unit activity        
Non-vested at the beginning of the period (in shares) 387      
Granted (in shares) 114      
Vested (in shares) (137)      
Cancelled (in shares) (20)      
Non-vested at the end of the period (in shares) 344 387    
Weighted-average fair value per share        
Non-vested at the beginning of the period (in dollars per share) $ 100.13      
Granted (in dollars per share) 128.76      
Vested (in dollars per share) 84.05      
Cancelled (in dollars per share) 114.98      
Non-vested at the end of the period (in dollars per share) $ 115.06 $ 100.13    
Other disclosures        
Fair value of awards vested during the year $ 15 $ 18 $ 15  
Unrecognized compensation cost $ 13      
Weighted-average period for amortization of unrecognized compensation cost 1 year 9 months 18 days      
Share-based payments subject to redemption $ 26 $ 25