Borrowing Arrangements - Amended and Restated Credit Facility (Details)		3 Months Ended	7 Months Ended	12 Months Ended	41 Months Ended
	Feb. 20, 2015 USD ($)	Mar. 31, 2018 USD ($)	Sep. 30, 2015	Sep. 30, 2016	Feb. 20, 2020	Oct. 02, 2012 USD ($)
Restated Credit Agreement
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 400,000,000.0
Senior credit facility, net of discount | Restated Credit Agreement
Debt Instrument [Line Items]
Maximum borrowing capacity	200,000,000.00
Current borrowing capacity		$ 144,400,000
Letters of credit outstanding		55,600,000
Borrowings excluding debt discount		145,000,000
Discount on debt		700,000
Deferred financing cost		900,000
Senior credit facility, net of discount | Senior Credit Facility
Debt Instrument [Line Items]
Maximum borrowing capacity						$ 450,000,000.0
Letter of credit facility | Restated Credit Agreement
Debt Instrument [Line Items]
Maximum borrowing capacity	100,000,000.00
Outstanding borrowing	53,400,000
Letter of credit facility | Senior Credit Facility
Debt Instrument [Line Items]
Maximum borrowing capacity						100,000,000.0
Swingline Loans | Restated Credit Agreement
Debt Instrument [Line Items]
Maximum borrowing capacity	20,000,000.0
Term loan facility | Restated Credit Agreement
Debt Instrument [Line Items]
Maximum borrowing capacity	200,000,000.0
Amount borrowed	10,400,000
Year one amortization of principal	15,000,000.0
Year two amortization of principal	15,000,000.0
Year three amortization of principal	20,000,000.0
Year four amortization of principal	20,000,000.0
Year five amortization of principal	20,000,000.0
Year six amortization of principal	110,000,000.0
Term loan facility | Senior Credit Facility
Debt Instrument [Line Items]
Maximum borrowing capacity						250,000,000.0
Revolving credit facility | Restated Credit Agreement
Debt Instrument [Line Items]
Maximum borrowing capacity	100,000,000.0
Outstanding borrowing	$ 147,300,000
Revolving credit facility | Senior Credit Facility
Debt Instrument [Line Items]
Maximum borrowing capacity						$ 200,000,000.0
LIBOR Loans | Senior credit facility, net of discount | Restated Credit Agreement
Debt Instrument [Line Items]
Period of total debt to EBITDA ratio	12 months
Base rate loans | Senior credit facility, net of discount | Restated Credit Agreement | Federal funds
Debt Instrument [Line Items]
Interest rate margin on variable rate basis (as a percent)	0.50%
Base rate loans | Senior credit facility, net of discount | Restated Credit Agreement | LIBOR
Debt Instrument [Line Items]
Interest rate margin on variable rate basis (as a percent)	1.00%
Credit Agreement | Senior credit facility, net of discount | Restated Credit Agreement
Debt Instrument [Line Items]
Maximum borrowing capacity		$ 144,400,000.0
Maximum | Senior credit facility, net of discount | Restated Credit Agreement
Debt Instrument [Line Items]
Total debt to EBITDA ratio that is required to be maintained			4.0	3.75
Minimum | Senior credit facility, net of discount | Restated Credit Agreement
Debt Instrument [Line Items]
Fixed charge coverage ratio that is required to be maintained	1.25
Forecast | Maximum | Senior credit facility, net of discount | Restated Credit Agreement
Debt Instrument [Line Items]
Total debt to EBITDA ratio that is required to be maintained					3.5