Long-Term Debt and Credit Facility - First Lien Credit Facility (Details) $ in Thousands		9 Months Ended	12 Months Ended
	Aug. 17, 2015 USD ($)	Sep. 30, 2019 USD ($) letter	Dec. 31, 2018 USD ($)
Long-term Debt
Fee on outstanding letters of credit (as a percent)		4.50%	4.50%
Fronting fee for letters of credit (as a percent)		0.125%	0.125%
Number of outstanding letters of credit | letter		1
Letters of credit outstanding		$ 50
Accrued interest		$ 4,834	$ 5,058
First Lien Credit Facility
Long-term Debt
Maximum net leverage ratio	7.00
Eurodollar rate | First Lien Credit Facility
Long-term Debt
Applicable margin over reference rate (as a percent)		4.50%
Eurodollar rate | First Lien Credit Facility | Minimum
Long-term Debt
Interest rate (as a percent)		1.00%
Base rate | First Lien Credit Facility
Long-term Debt
Applicable margin over reference rate (as a percent)		3.50%
Term Loan | First Lien Credit Facility
Long-term Debt
Maximum borrowing capacity	$ 435,000
Revolving Credit Facility
Long-term Debt
Amount outstanding		$ 0
Remaining borrowing capacity		$ 19,950
Revolving Credit Facility | Minimum
Long-term Debt
Unused commitment fee (as a percent)		0.375%	0.375%
Revolving Credit Facility | Maximum
Long-term Debt
Unused commitment fee (as a percent)		0.50%	0.50%
Revolving Credit Facility | First Lien Credit Facility
Long-term Debt
Maximum borrowing capacity	$ 40,000
Amount drawn to trigger net leverage requirement (as a percent)		50.00%
Revolving Credit Facility | Base rate
Long-term Debt
Applicable margin over reference rate (as a percent)		3.50%