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]
Tax cuts and jobs act of 2017, reclassification from AOCI to retained earnings, tax effect			0		0
Net income	$ 157,487		157,487
Cash dividends on common stock			(30,000)
Net loss on derivative instruments	0				0
Reclassifications of net losses on derivatives instruments	1,184				1,184
Reclassifications of benefits plans actuarial losses and net prior service costs	440				440
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	534,160	473,580	(25,979)
Balance, beginning of period at Dec. 31, 2017		60,259	534,160	473,580	(25,979)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Tax cuts and jobs act of 2017, reclassification from AOCI to retained earnings, tax effect			0		0
Net income	89,184		89,184
Cash dividends on common stock			(15,000)
Net loss on derivative instruments	0				0
Reclassifications of net losses on derivatives instruments	592				592
Reclassifications of benefits plans actuarial losses and net prior service costs	220				220
Balance, end of period		60,259	534,160	473,580	(25,979)
Balance, end of period	$ 1,117,016	60,259	608,344	473,580	(25,167)
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]
Tax cuts and jobs act of 2017, reclassification from AOCI to retained earnings, tax effect			5,451		(5,451)
Net income	$ 132,748		132,748
Cash dividends on common stock			(10,000)
Net loss on derivative instruments	(1,819)				(1,819)
Reclassifications of net losses on derivatives instruments	1,239				1,239
Reclassifications of benefits plans actuarial losses and net prior service costs	265				265
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	626,191	473,580	(26,226)
Balance, beginning of period at Dec. 31, 2018		60,259	626,191	473,580	(26,226)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Tax cuts and jobs act of 2017, reclassification from AOCI to retained earnings, tax effect			0		0
Net income	82,860		82,860
Cash dividends on common stock			(5,000)
Net loss on derivative instruments	(621)				(621)
Reclassifications of net losses on derivatives instruments	619				619
Reclassifications of benefits plans actuarial losses and net prior service costs	133				133
Balance, end of period		60,259	626,191	473,580	(26,226)
Balance, end of period	$ 1,211,795	$ 60,259	$ 704,051	$ 473,580	$ (26,095)
Common stock, par value (in usd per share)	$ 2.25