Long-Term Debt and Credit Facilities - Credit Facility (Details) - USD ($) $ in Thousands				1 Months Ended		4 Months Ended	9 Months Ended
	Jun. 30, 2018	Jun. 14, 2018	May 25, 2018	May 31, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	Apr. 03, 2018	Dec. 31, 2017	Oct. 24, 2017	Dec. 22, 2016
Long-term debt
Amount of deferred finance cost recognized						$ 13,656	$ 13,656			$ 19,679
Repayment of debt							$ 743,342	$ 645,478
Deferred purchase price
Long-term debt
Repayment of debt				$ 57,400
Total debt amount					$ 70,000			$ 70,000
Interest rate (as a percent)					5.00%			5.00%
Quarterly principal payments					$ 5,000
Proceeds from the IPO and funds drawn from the revolving credit facility				$ 4,800
Senior Secured Credit Facilities | Credit Facility
Long-term debt
Effective interest rate (as a percent)						7.00%	7.00%
First lien senior secured credit facility
Long-term debt
Maximum borrowing capacity												$ 670,000
Quarterly principal payments	$ 1,600
First lien senior secured credit facility | Credit Facility
Long-term debt
Maximum borrowing capacity											$ 135,000	100,000
Amount of deferred finance cost recognized		$ 1,200
Increase in face amount		65,000
Total debt amount		$ 200,000
First lien senior secured credit facility | Term loan
Long-term debt
Additional borrowing capacity									$ 95,000
Maximum borrowing capacity									665,000			570,000
Amount of deferred finance cost recognized									$ 900
Effective interest rate (as a percent)						5.49%	5.49%
Second lien senior secured credit facility | Term loan
Long-term debt
Maximum borrowing capacity												$ 175,000
Repayment of debt			$ 178,200			$ 178,200
Accrued interest paid			1,500
Prepayment penalty			$ 1,800