Financing (Narrative) (Details) - USD ($)	3 Months Ended		6 Months Ended
	Mar. 31, 2018	Apr. 01, 2017	Mar. 31, 2018	Apr. 01, 2017	Sep. 30, 2017
Line of Credit Facility [Line Items]
Percent added to Adjusted LIBOR rate for one month interest period			1.00%
Commitment fees incurred on credit facility	$ 77,000	$ 114,000	$ 156,000	$ 215,000
Annual excess cash flow percentage per credit agreement			50.00%
Net cash proceed percentage per credit agreement			100.00%
Alternate base rate
Line of Credit Facility [Line Items]
Effective interest rate applicable to outstanding credit facility (in hundredths)	2.25%		2.25%
Minimum
Line of Credit Facility [Line Items]
Line of credit facility, commitment fees payable, percent			0.25%
Minimum | Alternate base rate
Line of Credit Facility [Line Items]
Effective interest rate applicable to outstanding credit facility (in hundredths)	1.75%		1.75%
Maximum
Line of Credit Facility [Line Items]
Line of credit facility, commitment fees payable, percent			0.40%
Revolving credit facility
Line of Credit Facility [Line Items]
Line of credit facility, borrowing capacity	$ 120,000,000		$ 120,000,000
Maximum borrowing capacity	120,000,000		120,000,000
Short-term borrowings	0		0		$ 0
Letters of credit outstanding, amount	27,828,000		27,828,000		37,811,000
Line of credit facility, remaining borrowing capacity	92,172,000		$ 92,172,000		$ 82,189,000
Revolving credit facility | London interbank offered rate (LIBOR)
Line of Credit Facility [Line Items]
Basis spread on variable rate			0.50%
Term loan B facility
Line of Credit Facility [Line Items]
Line of credit facility, borrowing capacity	$ 460,000,000		$ 460,000,000
Principal amount, amortization percentage	1.00%		1.00%		1.00%
Weighted average interest rate	4.69%		4.69%
Eurocurrency term loans
Line of Credit Facility [Line Items]
Effective interest rate applicable to outstanding credit facility (in hundredths)	3.25%		3.25%
Eurocurrency term loans | London interbank offered rate (LIBOR)
Line of Credit Facility [Line Items]
Effective interest rate applicable to outstanding credit facility (in hundredths)	1.74%		1.74%
Eurocurrency term loans | Minimum | London interbank offered rate (LIBOR)
Line of Credit Facility [Line Items]
Effective interest rate applicable to outstanding credit facility (in hundredths)	0.75%		0.75%
Eurocurrency borrowing | London interbank offered rate (LIBOR)
Line of Credit Facility [Line Items]
Effective interest rate applicable to outstanding credit facility (in hundredths)	3.00%		3.00%