Debt (Details) - USD ($)		3 Months Ended		9 Months Ended
	Jul. 31, 2015	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Jan. 31, 2019	Dec. 31, 2018
Credit facility
Long term debt, gross		$ 407,841,000		$ 407,841,000
Amortization of deferred financing costs				$ 2,032,000	$ 1,842,000
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		376,400,000		376,400,000
Debt issuance costs incurred	$ 12,900,000	12,900,000		12,900,000
Deferred issuance costs, net		4,474,000		4,474,000			$ 6,168,000
Face amount	$ 425,000,000.0
Amortization per year (as a percent)	1.00%
Long term debt, gross		406,900,000		406,900,000
Amortization of deferred financing costs		600,000	$ 600,000	1,700,000	$ 1,800,000
Term loan | Prepayment due to Excess Cash Flow provision
Credit facility
Repayments of principal in next 12 months		0		0
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%