Long-term Debt (Schedule of Long-term Debt) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016
Total debt	$ 721,852	$ 729,244
Current portion of long-term debt	(150,000)	(150,000)
Unamortized debt issuance costs	(1,974)	(2,091)
Long-term debt, less current portion	$ 569,878	$ 577,153
Series D Notes [Member] | Notes Payable to Banks [Member]
Interest rate	6.39%	6.39%
Maturity date	Oct. 01, 2018	Oct. 01, 2018
Total debt	$ 100,000	$ 100,000
Series F Notes [Member] | Notes Payable to Banks [Member]
Interest rate	8.24%	8.24%
Maturity date	Apr. 03, 2019	Apr. 03, 2019
Total debt	$ 43,000	$ 43,000
Series J Notes [Member]
Variable interest rate	2.16%
Series J Notes [Member] | Notes Payable to Banks [Member]
Basis Spread On Variable Rate	1.25%	1.25%
Maturity date	Nov. 15, 2020	Nov. 15, 2020
Total debt	$ 50,000	$ 50,000
Series G Notes [Member] | Notes Payable to Banks [Member]
Interest rate	3.42%	3.42%
Maturity date	Nov. 15, 2020	Nov. 15, 2020
Total debt	$ 50,000	$ 50,000
Series H Notes [Member] | Notes Payable to Banks [Member]
Interest rate	4.03%	4.03%
Maturity date	Nov. 15, 2023	Nov. 15, 2023
Total debt	$ 25,000	$ 25,000
Series I Notes [Member] | Notes Payable to Banks [Member]
Interest rate	4.18%	4.18%
Maturity date	Nov. 15, 2025	Nov. 15, 2025
Total debt	$ 25,000	$ 25,000
Series K Notes [Member] | Notes Payable to Banks [Member]
Interest rate	4.03%	4.03%
Maturity date	Nov. 15, 2023	Nov. 15, 2023
Total debt	$ 50,000	$ 50,000
Series L Notes [Member] | Notes Payable to Banks [Member]
Interest rate	4.18%	4.18%
Maturity date	Nov. 15, 2025	Nov. 15, 2025
Total debt	$ 50,000	$ 50,000
Series M Notes [Member] | Notes Payable to Banks [Member]
Interest rate	1.12%	1.12%
Maturity date	Sep. 23, 2026	Sep. 23, 2026
Total debt	$ 42,076	$ 44,886
Series N Notes [Member] | Notes Payable to Banks [Member]
Interest rate	1.31%	1.31%
Maturity date	Sep. 23, 2028	Sep. 23, 2028
Total debt	$ 80,995	$ 86,406
Series O Notes [Member] | Notes Payable to Banks [Member]
Interest rate	1.57%	1.57%
Maturity date	Sep. 23, 2031	Sep. 23, 2031
Total debt	$ 45,231	$ 48,252
Revolving Credit Agreement [Member]
Variable Rate Basis	LIBOR
Credit facility effective interest rate on outstanding borrowing	1.66%	1.77%
Revolving Credit Agreement [Member] | Domestic Line Of Credit [Member]
Total debt	$ 160,550	$ 156,700
Revolving Credit Agreement [Member] | Minimum [Member]
Basis Spread On Variable Rate	0.85%
Revolving Credit Agreement [Member] | Maximum [Member]
Basis Spread On Variable Rate	1.65%
Chinese Credit Facility, RMB Denominated Loan [Member]
Variable Rate Basis	prevailing interest rate offered by the People's Bank of China on the date of borrowing, plus a margin equal to 15% of that prevailing rate	interest at the prevailing interest rate offered by the People's Bank of China on the date of borrowing, plus a margin equal to 15% of that prevailing rate
Chinese Credit Facility, USD Denominated Loan [Member]
Variable Rate Basis	lender's cost of borrowing rate at the date of borrowing, plus 3%	interest at the lender's cost of borrowing rate at the date of borrowing, plus 3%