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Equity - Performance shares (Details)
$ / shares in Units, shares in Thousands, $ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2019
Dec. 31, 2019
USD ($)
item
$ / shares
shares
Dec. 31, 2018
USD ($)
$ / shares
shares
Dec. 31, 2017
USD ($)
$ / shares
shares
Dec. 31, 2016
USD ($)
Share-based compensation          
Share-based payments subject to redemption   $ 31 $ 37 $ 36 $ 30
Performance Shares          
Share-based compensation          
Number of tranches | item   2      
Vesting terms   3 years      
Performance Shares | Long Term Incentive Plan          
Share-based compensation          
Granted (in shares) | shares   70 27 38  
Weighted-average fair value per share, Granted (in dollars per share) | $ / shares   $ 92.57 $ 141.91 $ 114.08  
Unrecognized compensation cost   $ 4      
Remaining requisite service period (in years)   1 year 9 months 18 days      
Cancelled (in shares) | shares   3      
Share-based payments subject to redemption   $ 9 $ 10    
Market-based performance shares          
Share-based compensation          
Percentage of share-based compensation award   50.00%      
Performance shares calculation period (in years)   3 years      
Vesting terms   3 years      
Market-based performance shares | Minimum          
Share-based compensation          
Performance shares available for vesting (as a percent)   0.00%      
Market-based performance shares | Maximum          
Share-based compensation          
Performance shares available for vesting (as a percent)   200.00%      
Internal-based performance based          
Share-based compensation          
Percentage of share-based compensation award   50.00%      
Performance shares calculation period (in years)   3 years      
Vesting terms   3 years      
Internal-based performance based | Minimum          
Share-based compensation          
Performance shares available for vesting (as a percent)   0.00%      
Internal-based performance based | Maximum          
Share-based compensation          
Performance shares available for vesting (as a percent)   200.00%      
Performance Shares Award Granted in 2017          
Share-based compensation          
Estimated pay out (as a percent)   0.00%      
Performance Shares Award Granted in 2016 | Long Term Incentive Plan          
Share-based compensation          
Award pay out achieved (as a percent) 0.00%