Borrowings - Fourth Amended and Restated Credit Agreement (Details) - USD ($) $ in Thousands				12 Months Ended			18 Months Ended
	Sep. 19, 2018	Jan. 02, 2018	Nov. 03, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Jun. 30, 2017
Debt Instrument [Line Items]
Outstanding principal amount				$ 931,908			$ 931,908
Repayments of long-term debt				500,000	$ 256,473	$ 3,825
LIBOR
Debt Instrument [Line Items]
Interest rate margin (as a percent)			1.25%
Senior Secured Credit Facility Fourth Amendment
Debt Instrument [Line Items]
Outstanding principal amount				400,000			400,000	$ 1,150,000
Repayments of long-term debt				500,000			$ 750,000
Accelerated unamortized financing costs				$ 10,600
Senior Secured Credit Facility Fourth Amendment | LIBOR
Debt Instrument [Line Items]
Interest rate margin (as a percent)	2.75%	3.25%