Debt (Details) - USD ($)		3 Months Ended	9 Months Ended		12 Months Ended
	Jul. 31, 2015	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2016	Nov. 18, 2015
Credit facility
Long term debt, gross		$ 415,399,000	$ 415,399,000
Amortization of deferred financing costs			1,842,000	$ 1,764,000
Excess cash flows, repayment of debt, percentage					50.00%
Excess cash flows threshold for dividends					$ 5,000,000
Term loan
Credit facility
Fair value of debt		410,700,000	410,700,000
Debt issuance costs incurred	$ 12,900,000	12,900,000	12,900,000
Deferred issuance costs, net		6,700,000	6,700,000
Face amount	$ 425,000,000
Amortization per year (as a percent)	1.00%
Long term debt, gross		413,300,000	413,300,000
Amortization of deferred financing costs		600,000	1,800,000
Revolving loan
Credit facility
Debt issuance costs incurred	$ 1,300,000
Maximum borrowing available	25,000,000
Long term debt, gross		0	0
Letter-of-credit sub-facility
Credit facility
Maximum borrowing available	$ 10,000,000	500,000	500,000
Letters of credit, amount outstanding		0	0
Credit Facility
Credit facility
Maximum dividends paid and shares and warrants repurchased per fiscal year						$ 12,000,000
Aggregate purchase price of warrants						$ 10,000,000
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