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Equity - Performance shares (Details)
$ / shares in Units, shares in Thousands, $ in Millions
6 Months Ended 12 Months Ended
Jun. 30, 2020
USD ($)
item
$ / shares
shares
Dec. 31, 2018
USD ($)
Mar. 31, 2020
USD ($)
Dec. 31, 2019
USD ($)
Jun. 30, 2019
USD ($)
Mar. 31, 2019
USD ($)
Share-based compensation            
Share-based payments subject to redemption | $ $ 27.0 $ 37.0 $ 23.0 $ 31.0 $ 25.0 $ 21.0
Performance Shares            
Share-based compensation            
Number of tranches | item 2          
Vesting terms   3 years        
Granted (in shares) | shares 81          
Weighted-average fair value per share, Granted (in dollars per share) | $ / shares $ 94.48          
Unrecognized compensation cost | $ $ 8.0          
Remaining requisite service period (in years) 2 years 2 months 12 days          
Cancelled (in shares) | shares 0          
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            
Award pay out achieved (as a percent) 0.00%