Condensed Consolidated Statement Of Changes In Equity Statement - USD ($) $ in Thousands	Total	Common Stock	Retained Earnings	Additional Paid-in Capital	AOCI Including Portion Attributable to Noncontrolling Interest
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of change in accounting for employee share-based payments			$ 0
Balance, end of period		$ 60,259	480,857	$ 473,580	$ (26,791)
Balance, beginning of period at Sep. 30, 2017		60,259	480,857	473,580	(26,791)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Reclassification of stranded income tax effects related to TCJA			0		0
Net income	$ 154,464		154,464
Cash dividends on common stock			(45,000)
Net losses on derivative instruments	0				0
Reclassifications of net losses on derivative instruments	1,776				1,776
Reclassifications of benefit plans actuarial losses and net prior service credits	660				660
Balance, end of period		60,259	480,857	473,580	(26,791)
Cumulative effect of change in accounting for employee share-based payments			0
Balance, end of period		60,259	608,344	473,580	(25,167)
Balance, beginning of period at Mar. 31, 2018		60,259	608,344	473,580	(25,167)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Reclassification of stranded income tax effects related to TCJA			0		0
Net income	(3,023)		(3,023)
Cash dividends on common stock			(15,000)
Net losses on derivative instruments	0				0
Reclassifications of net losses on derivative instruments	592				592
Reclassifications of benefit plans actuarial losses and net prior service credits	220				220
Balance, end of period		60,259	608,344	473,580	(25,167)
Balance, end of period	$ 1,099,805	60,259	590,321	473,580	(24,355)
Common stock, par value (in usd per share)	$ 2.25
Balance, beginning of period at Sep. 30, 2018		60,259	579,778	473,580	(20,329)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, end of period		60,259	579,778	473,580	(20,329)
Cumulative effect of change in accounting for employee share-based payments			(3,926)
Balance, end of period		60,259	579,778	473,580	(20,329)
Common stock, par value (in usd per share)	$ 2.25
Balance, beginning of period at Sep. 30, 2018		60,259	579,778	473,580	(20,329)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Reclassification of stranded income tax effects related to TCJA			5,451		(5,451)
Net income	$ 139,421		139,421
Cash dividends on common stock			(15,000)
Net losses on derivative instruments	(3,016)				(3,016)
Reclassifications of net losses on derivative instruments	1,859				1,859
Reclassifications of benefit plans actuarial losses and net prior service credits	397				397
Balance, end of period		60,259	579,778	473,580	(20,329)
Cumulative effect of change in accounting for employee share-based payments			0
Balance, end of period		60,259	704,051	473,580	(26,095)
Balance, beginning of period at Mar. 31, 2019		60,259	704,051	473,580	(26,095)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Reclassification of stranded income tax effects related to TCJA			0		0
Net income	6,673		6,673
Cash dividends on common stock			(5,000)
Net losses on derivative instruments	(1,197)				(1,197)
Reclassifications of net losses on derivative instruments	620				620
Reclassifications of benefit plans actuarial losses and net prior service credits	132				132
Balance, end of period		60,259	704,051	473,580	(26,095)
Balance, end of period	$ 1,213,023	$ 60,259	$ 705,724	$ 473,580	$ (26,540)
Common stock, par value (in usd per share)	$ 2.25