Long-term Debt (Schedule of Long-term Debt) (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended	12 Months Ended
Mar. 31, 2018	Mar. 31, 2018	Sep. 30, 2017
Debt Instrument [Line Items]
Unamortized debt issuance costs	$ (1,654)	$ (1,654)	$ (1,794)
Long-term debt, excluding current portion	588,461	588,461	580,286
Notes Payable to Banks [Member]
Debt Instrument [Line Items]
Long-term Debt	588,461	588,461	580,286
Current portion of long-term debt	0	0	0
Long-term debt, excluding current portion	$ 588,461	$ 588,461	$ 580,286
Series D Notes [Member] \| Notes Payable to Banks [Member]
Debt Instrument [Line Items]
Interest rate	6.39%	6.39%	6.39%
Maturity date		Oct. 01, 2018	Oct. 01, 2018
Long-term Debt	$ 100,000	$ 100,000	$ 100,000
Series F Notes [Member] \| Notes Payable to Banks [Member]
Debt Instrument [Line Items]
Interest rate	8.24%	8.24%	8.24%
Maturity date		Apr. 03, 2019	Apr. 03, 2019
Long-term Debt	$ 43,000	$ 43,000	$ 43,000
Series J Notes [Member]
Debt Instrument [Line Items]
Variable interest rate	3.12%	3.12%
Variable interest rate	3.12%	3.12%
Series J Notes [Member] \| Notes Payable to Banks [Member]
Debt Instrument [Line Items]
Basis Spread On Variable Rate		1.25%	1.25%
Maturity date		Nov. 15, 2020	Nov. 15, 2020
Long-term Debt	$ 50,000	$ 50,000	$ 50,000
Series G Notes [Member] \| Notes Payable to Banks [Member]
Debt Instrument [Line Items]
Interest rate	3.42%	3.42%	3.42%
Maturity date		Nov. 15, 2020	Nov. 15, 2020
Long-term Debt	$ 50,000	$ 50,000	$ 50,000
Series H Notes [Member] \| Notes Payable to Banks [Member]
Debt Instrument [Line Items]
Interest rate	4.03%	4.03%	4.03%
Maturity date		Nov. 15, 2023	Nov. 15, 2023
Long-term Debt	$ 25,000	$ 25,000	$ 25,000
Series I Notes [Member] \| Notes Payable to Banks [Member]
Debt Instrument [Line Items]
Interest rate	4.18%	4.18%	4.18%
Maturity date		Nov. 15, 2025	Nov. 15, 2025
Long-term Debt	$ 25,000	$ 25,000	$ 25,000
Series K Notes [Member] \| Notes Payable to Banks [Member]
Debt Instrument [Line Items]
Interest rate	4.03%	4.03%	4.03%
Maturity date		Nov. 15, 2023	Nov. 15, 2023
Long-term Debt	$ 50,000	$ 50,000	$ 50,000
Series L Notes [Member] \| Notes Payable to Banks [Member]
Debt Instrument [Line Items]
Interest rate	4.18%	4.18%	4.18%
Maturity date		Nov. 15, 2025	Nov. 15, 2025
Long-term Debt	$ 50,000	$ 50,000	$ 50,000
Series M Notes [Member] \| Notes Payable to Banks [Member]
Debt Instrument [Line Items]
Interest rate	1.12%	1.12%	1.12%
Maturity date		Sep. 23, 2026	Sep. 23, 2026
Long-term Debt	$ 49,279	$ 49,279	$ 47,270
Series N Notes [Member] \| Notes Payable to Banks [Member]
Debt Instrument [Line Items]
Interest rate	1.31%	1.31%	1.31%
Maturity date		Sep. 23, 2028	Sep. 23, 2028
Long-term Debt	$ 94,861	$ 94,861	$ 90,995
Series O Notes [Member] \| Notes Payable to Banks [Member]
Debt Instrument [Line Items]
Interest rate	1.57%	1.57%	1.57%
Maturity date		Sep. 23, 2031	Sep. 23, 2031
Long-term Debt	$ 52,975	$ 52,975	$ 50,815
Revolving Credit Agreement [Member]
Debt Instrument [Line Items]
Variable Rate Basis	LIBOR
Revolving Credit Agreement [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Basis Spread On Variable Rate	0.85%
Revolving Credit Agreement [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Basis Spread On Variable Rate	1.65%

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

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- Definition

The reference rate for the variable rate of the debt instrument, such as LIBOR or the US Treasury rate and the maturity of the reference rate used, such as three months or six months LIBOR.

+ References

No definition available.

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Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

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Contractual interest rate for funds borrowed, under the debt agreement.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

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Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28555-108399

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt, classified as current. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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