Borrowing Arrangements - Amended and Restated Credit Facility (Details)		1 Months Ended		7 Months Ended	9 Months Ended		12 Months Ended	41 Months Ended
	Feb. 20, 2015 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Sep. 30, 2015	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016	Feb. 20, 2020	Dec. 31, 2017 USD ($)
Debt Instrument [Line Items]
Debt repayment					$ 101,600,000	$ 308,700,000
Long-term borrowings		$ 107,800,000			107,800,000				$ 153,800,000
Restated Credit Agreement
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 400,000,000.0
Debt repayment		20,000,000	$ 15,000,000
Senior credit facility, net of discount
Debt Instrument [Line Items]
Long-term borrowings		105,200,000			105,200,000				$ 151,000,000
Senior credit facility, net of discount | Restated Credit Agreement
Debt Instrument [Line Items]
Maximum borrowing capacity	200,000,000.00
Current borrowing capacity		145,900,000			145,900,000
Letters of credit outstanding		48,100,000			48,100,000
Discount on debt		400,000			400,000
Deferred financing cost					500,000
Letter of credit facility | Restated Credit Agreement
Debt Instrument [Line Items]
Maximum borrowing capacity	100,000,000.00
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
Year one amortization of principal	15,000,000
Year two amortization of principal	15,000,000
Year three amortization of principal	20,000,000
Year four amortization of principal	20,000,000
Year five amortization of principal	20,000,000
Year six amortization of principal	110,000,000
Revolving credit facility | Restated Credit Agreement
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 100,000,000.0
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	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	1.00%
Credit Agreement | Senior credit facility, net of discount | Restated Credit Agreement
Debt Instrument [Line Items]
Maximum borrowing capacity		$ 145,920,000.00			$ 145,920,000.00
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