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, 2015		$ 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 gains (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 benefits	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 gains (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 benefits	956				956
Balance, end of year at Sep. 30, 2017	987,905	60,259	480,857	473,580	(26,791)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	148,921		148,921
Cash dividends — Common Stock			(50,000)
Excess tax benefits on equity-based compensation				0
Net gains (losses) on derivative instruments	20				20
Reclassifications of net losses on derivative instruments	2,367				2,367
Benefit plans, principally actuarial gains (losses)					3,203
Reclassifications of benefit plans actuarial losses and net prior service benefits	872				872
Balance, end of year at Sep. 30, 2018	$ 1,093,288	$ 60,259	$ 579,778	$ 473,580	$ (20,329)