Related Party Transactions - Additional Information (Details) - USD ($) $ in Thousands		1 Months Ended	9 Months Ended	12 Months Ended
	Jul. 02, 2018	Aug. 31, 2018	Sep. 30, 2017	Sep. 30, 2018	Dec. 31, 2016
Monitoring Agreement
Related Party Transaction [Line Items]
Percentage of advisory fee equal to adjusted EBITDA				1.00%
IPO
Related Party Transaction [Line Items]
Amount of underwriter discount and commissions received	$ 37,717
Kohlberg Kravis Roberts Capital Markets L L C | Monitoring Agreement
Related Party Transaction [Line Items]
Management and/or advisory fees paid			$ 1,486	$ 1,633	$ 2,105
Kohlberg Kravis Roberts Capital Markets L L C | IPO | Monitoring Agreement
Related Party Transaction [Line Items]
Termination fee paid				7,598
MSD Partners (“MSD”) | Monitoring Agreement
Related Party Transaction [Line Items]
Management and/or advisory fees paid				463	$ 611
MSD Partners (“MSD”) | IPO | Monitoring Agreement
Related Party Transaction [Line Items]
Termination fee paid				3,438
MSD Capital L.P. | Monitoring Agreement
Related Party Transaction [Line Items]
Management and/or advisory fees paid			$ 431
Affiliated Entity | Kohlberg Kravis Roberts Capital Markets L L C
Related Party Transaction [Line Items]
Debt refinancing transaction cost		$ 1,670
Affiliated Entity | Kohlberg Kravis Roberts Capital Markets L L C | IPO
Related Party Transaction [Line Items]
Amount of underwriter discount and commissions received				$ 5,497