Quarterly report pursuant to Section 13 or 15(d)

Equity-Based Compensation - PSU awards (Details)

v3.10.0.1
Equity-Based Compensation - PSU awards (Details) - USD ($)
$ / shares in Units, $ in Millions
6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Performance share unit awards    
Number of units    
Total granted and unvested at the beginning of the period (in shares) 1,283,843  
Granted (in shares) 756,466  
Vested (in shares) (41,666)  
Forfeited (in shares) (27,166)  
Total awarded and unvested at the end of the period (in shares) 1,971,477  
Weighted average grant date fair value    
Total granted and unvested at the beginning of the period (in dollars per share) $ 28.29  
Granted (in dollars per share) 23.61  
Vested (in dollars per share) 27.38  
Forfeited (in dollars per share) 30.15  
Total awarded and unvested at the end of the period (in dollars per share) $ 26.49  
Additional disclosures    
Additional equity compensation to be recognized over the remaining period $ 29.0  
Weighted average period for recognizing unrecognized stock-based compensation expense 2 years 2 months 12 days  
Weighted-average assumptions used to calculate fair value of performance share units granted    
Weighted average fair value of awards granted (in dollars per share) $ 23.61  
Price target and TSR performance share unit awards    
Price target as a percentage of beginning price 125.00%  
Vesting period 3 years  
Amortization period of PSU expense 3 years  
Price target and TSR performance share unit awards | Minimum    
Number of PSUs that may be earned as compared to the number of PSUs granted, as a percent 0.00%  
Price target and TSR performance share unit awards | Maximum    
Number of PSUs that may be earned as compared to the number of PSUs granted, as a percent 200.00%  
Price target performance share unit awards    
Number of successive days closing stock price must achieve specific thresholds for PSUs to vest per schedule 10 days  
Vesting period 3 years  
Price target performance share unit awards | Vesting before first anniversary | Maximum    
Number of PSUs that may vest, as a percent 0.00%  
Price target performance share unit awards | Vesting before the second anniversary | Maximum    
Number of PSUs that may vest, as a percent 33.33%  
Price target performance share unit awards | Vesting before the third anniversary | Maximum    
Number of PSUs that may vest, as a percent 66.67%  
TSR performance share unit awards    
Vesting period 3 years  
TSR performance share unit awards | Minimum    
Number of PSUs that may be earned as compared to the number of PSUs granted, as a percent 0.00%  
TSR performance share unit awards | Maximum    
Number of PSUs that may be earned as compared to the number of PSUs granted, as a percent 200.00%  
ROCE performance share unit awards    
Vesting period 3 years  
ROCE performance share unit awards | Minimum    
Number of PSUs that may vest, as a percent 0.00%  
ROCE performance share unit awards | Maximum    
Number of PSUs that may vest, as a percent 200.00%  
Market-Based PSUs    
Weighted average grant date fair value    
Granted (in dollars per share) $ 24.85 $ 26.21
Weighted-average assumptions used to calculate fair value of performance share units granted    
Volatility (as a percent) 41.00% 42.00%
Risk-free interest rate (as a percent) 2.49% 1.40%
Weighted average fair value of awards granted (in dollars per share) $ 24.85 $ 26.21