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, 2014		$ 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 gains (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 gains (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)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	116,041		116,041
Cash dividends — Common Stock			(57,700)
Excess tax benefits on equity-based compensation				0
Net losses on derivative instruments	0				0
Reclassifications of net losses on derivative instruments	1,988				1,988
Benefit plans, principally actuarial gains (losses)					1,883
Reclassifications of benefit plans actuarial losses and net prior service credits	956				956
Balance, end of year at Sep. 30, 2017	$ 987,905	$ 60,259	$ 480,857	$ 473,580	$ (26,791)