Equity-Based Compensation - Nonvested Unit Options Outstanding (Details) - 2005 Common Membership Unit Plan - $ / shares shares in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Number of Nonvested Options Outstanding
Nonvested options outstanding (in shares)	0	474	1,398
Options forfeited (in units)		(195)
Options vested (in units)		(279)	(924)
Nonvested options outstanding (in shares)	0	0	474
Nonvested options outstanding (in shares)	0	474	1,398	0	0	474	1,398
Weighted Average Grant Date Fair Value per Option
Nonvested unit options outstanding (in dollars per unit)				$ 0.00	$ 0.00	$ 0.97	$ 0.93
Options forfeited (in dollars per unit)		$ 0.97
Options vested (in dollars per unit)		$ 0.97	$ 0.90