Consolidated Statements of Stockholder's Equity - USD ($) $ in Thousands	Total	Common stock, without par value	Retained earnings	Additional paid-in capital	Accumulated other comprehensive income (loss)
Balance, beginning of year at Sep. 30, 2013		$ 60,259	$ 269,977	$ 470,098	$ (8,720)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 124,106		124,106
Cash dividends — Common Stock			(77,395)
Excess tax benefits on equity-based compensation				973
Net losses on derivative instruments	0				0
Reclassifications of net losses on derivative instruments	1,567				1,567
Benefit plans, principally actuarial losses					(1,413)
Reclassifications of benefit plans actuarial losses and net prior service credits	385				385
Balance, end of year at Sep. 30, 2014	839,837	60,259	316,688	471,071	(8,181)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	121,055		121,055
Cash dividends — Common Stock			(65,600)
Excess tax benefits on equity-based compensation				833
Net losses on derivative instruments	(4,105)				(4,105)
Reclassifications of net losses on derivative instruments	1,565				1,565
Benefit plans, principally actuarial losses					(3,482)
Reclassifications of benefit plans actuarial losses and net prior service credits	517				517
Balance, end of year at Sep. 30, 2015	890,620	60,259	372,143	471,904	(13,686)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	97,373		97,373
Cash dividends — Common Stock			(47,000)
Excess tax benefits on equity-based compensation				1,676
Net losses on derivative instruments	(16,942)				(16,942)
Reclassifications of net losses on derivative instruments	1,568				1,568
Benefit plans, principally actuarial losses					(3,197)
Reclassifications of benefit plans actuarial losses and net prior service credits	639				639
Balance, end of year at Sep. 30, 2016	$ 924,737	$ 60,259	$ 422,516	$ 473,580	$ (31,618)