Long-Term Debt - General Information (Details) - USD ($) $ in Thousands						1 Months Ended	12 Months Ended
	Mar. 28, 2018	Oct. 06, 2017	Mar. 17, 2017	Oct. 13, 2016	Sep. 21, 2016	Jun. 30, 2015	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Nov. 08, 2016	Nov. 20, 2013
Long-Term Debt
Amount outstanding							$ 1,165,865	$ 916,902
Cash distribution to members					$ 100,000		101,755	149,633	$ 236,145
Prior Senior Secured Credit Facility, Term Loan Facility
Long-Term Debt
Repayments of debt				$ 200,400
Term Loan Facility
Long-Term Debt
Amount outstanding							1,156,345	916,902
Unamortized discount							$ 5,400	$ 6,000
Secured Debt | Line of Credit | Prior Senior Secured Credit Facility
Long-Term Debt
Line of Credit Facility, Maximum Borrowing Capacity													$ 545,000
Secured Debt | Line of Credit | Prior Senior Secured Credit Facility, Term Loan Facility
Long-Term Debt
Line of Credit Facility, Maximum Borrowing Capacity					828,200								$ 525,000
Maximum borrowing capacity, increase in capacity					135,000	$ 95,000				$ 55,000	$ 117,000
Original issue discount from increase in capacity					$ 16,500
Original issue discount (as a percentage)													1.00%
Unamortized discount													$ 5,300
Secured Debt | Line of Credit | Senior Secured Credit Facility
Long-Term Debt
Line of Credit Facility, Maximum Borrowing Capacity			$ 740,000									$ 680,000
Maximum borrowing capacity, increase in capacity			$ 95,000
Secured Debt | Line of Credit | Term Loan Facility
Long-Term Debt
Line of Credit Facility, Maximum Borrowing Capacity	$ 1,190,000	$ 939,500										645,000
Interest rate (as a percent)							5.13%
Amount outstanding							$ 1,175,100
Maximum borrowing capacity, increase in capacity	$ 250,000	$ 205,000
Secured Debt | Revolving Credit Facility | Prior Senior Secured Credit Facility, Revolving Credit Facility
Long-Term Debt
Line of Credit Facility, Maximum Borrowing Capacity													$ 20,000
Secured Debt | Revolving Credit Facility | Revolving Credit Facility
Long-Term Debt
Maximum amount allocated to letters of credit												15,000
Line of Credit Facility, Maximum Borrowing Capacity												$ 35,000
Available borrowings							31,300
Amount outstanding							0
Secured Debt | Letters of credit | Senior Secured Credit Facility, Letters of Credit
Long-Term Debt
Available borrowings							$ 3,700