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Debt - Composition of Long Term Debt (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Sep. 30, 2017	Oct. 31, 2016
Debt Instrument
Total long-term debt	$ 842,130	$ 755,000
Capital Lease Obligations	6,817	0
Less: unamortized debt issuance costs	(4,134)	(3,899)
Less: current maturities	(9,001)	(39,996)
Total long-term debt due after one year	$ 828,995	711,105
Senior Notes \| 4.12%, due September 2046
Debt Instrument
Interest rate	4.12%
Total long-term debt	$ 200,000	200,000
Senior Notes \| 4.98%, due March 2044
Debt Instrument
Interest rate	4.98%
Total long-term debt	$ 175,000	175,000
Senior Notes \| 4.12%, due October 2046
Debt Instrument
Interest rate	4.12%		4.12%
Total long-term debt	$ 100,000	100,000
Senior Notes \| 6.21%, due September 2036
Debt Instrument
Interest rate	6.21%
Total long-term debt	$ 100,000	100,000
Senior Notes \| 2.95%, due June 2026
Debt Instrument
Interest rate	2.95%
Total long-term debt	$ 100,000	$ 100,000
Medium-term Notes \| 6.17%, due June 2017
Debt Instrument
Interest rate		6.17%
Medium-term Notes \| 7.25%, due November 2017
Debt Instrument
Interest rate	7.25%
Total long-term debt	$ 0	$ 20,000
Medium-term Notes \| 5.67%, due January 2018
Debt Instrument
Interest rate	5.67%
Total long-term debt	$ 0	20,000
Medium-term Notes \| 6.50%, due August 2033
Debt Instrument
Interest rate	6.50%
Total long-term debt	$ 20,000	20,000
Medium-term Notes \| 6.13%, due October 2034
Debt Instrument
Interest rate	6.13%
Total long-term debt	$ 20,000	20,000
Term Loan
Debt Instrument
Total long-term debt	$ 120,313	$ 0