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Equity-Based Compensation - PSU awards (Details) - USD ($)
$ / shares in Units, $ in Millions
9 Months Ended 12 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Dec. 31, 2018
Performance share unit awards      
Number of units      
Total awarded and unvested at the beginning of the period (in shares) 1,767,299    
Granted (in shares) 1,416,378    
Vested (in shares) (31,944)    
Forfeited (in shares) (614,450)    
Total awarded and unvested at the end of the period (in shares) 2,537,283   1,767,299
Weighted average grant date fair value      
Total awarded and unvested at the beginning of the period (in dollars per share) $ 26.36    
Granted (in dollars per share) 9.26 $ 24.85  
Vested (in dollars per share) 27.38    
Forfeited (in dollars per share) 26.61    
Total awarded and unvested at the end of the period (in dollars per share) $ 16.74   $ 26.36
Additional disclosures      
Additional equity compensation to be recognized over the remaining period $ 20    
Weighted average period for recognizing unrecognized stock-based compensation expense 1 year 10 months 24 days    
Weighted-average assumptions used to calculate fair value of performance share units granted      
Volatility (as a percent) 36.00% 41.00%  
Risk-free interest rate (as a percent) 2.35% 2.49%  
Weighted average fair value of awards granted (in dollars per share) $ 9.26 $ 24.85  
Price target and TSR performance share unit awards      
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      
Price target as a percentage of beginning price 125.00%    
Vesting period 3 years    
Amortization period of PSU expense 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    
Amortization period of PSU expense 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      
Price target as a percentage of beginning price 125.00%    
Number of PSUs that may vest, as a percent 200.00%