Debt (Details) - USD ($)		3 Months Ended			6 Months Ended
	Jul. 31, 2015	Jun. 30, 2019	Jun. 30, 2018	Mar. 31, 2017	Jun. 30, 2019	Jun. 30, 2018	Jan. 31, 2019
Credit facility
Long term debt, gross		$ 410,185,000			$ 410,185,000
Amortization of deferred financing costs					1,409,000	$ 1,214,000
Excess cash flows, repayment of debt, percentage				50.00%
Excess cash flows threshold for dividends				$ 5,000,000.0
Term loan
Credit facility
Fair value of debt		384,800,000			384,800,000
Debt issuance costs incurred	$ 12,900,000	12,900,000			12,900,000
Deferred issuance costs, net		5,100,000			5,100,000
Face amount	$ 425,000,000.0
Amortization per year (as a percent)	1.00%
Long term debt, gross		409,100,000			409,100,000
Amortization of deferred financing costs		1,100,000	$ 600,000		1,200,000
Revolving loan
Credit facility
Debt issuance costs incurred	$ 1,300,000
Maximum borrowing available	$ 25,000,000.0						$ 12,500,000
Long term debt, gross		0			0
Letter-of-credit sub-facility
Credit facility
Maximum borrowing available							$ 10,000,000.0
Credit Facility | Alternate base rate
Credit facility
Margin of interest (as a percent)	3.75%
Credit Facility | LIBOR
Credit facility
Margin of interest (as a percent)	4.75%
Variable rate base minimum (as a percent)	1.00%
Prepayment due to Excess Cash Flow provision | Term loan
Credit facility
Repayments of principal in next 12 months		$ 0			$ 0