CONSOLIDATED STATEMENTS OF COMMON STOCK EQUITY - USD ($) $ in Thousands		Total	Common Stock	Premium on Common Stock	Accumulated Other Comprehensive (Loss) Income	Treasury Stock And Other	Retained Earnings
Beginning Balance (in shares) at Sep. 30, 2016			86,086,000
Beginning Balance at Sep. 30, 2016		$ 1,166,591	$ 221,654	$ 215,580	$ (15,155)	$ (81,044)	$ 825,556
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		132,065					132,065
Other comprehensive income (loss)		11,899			11,899
Common stock issued:
Incentive compensation plan (in shares)			241,000
Incentive compensation plan		5,694	$ 604	5,090
Dividend reinvestment plan (in shares)	[1]		472,000
Dividend reinvestment plan	[1]	17,622		(946)		18,568
Cash dividend declared ($1.19 per share in 2019, $1.11 per share in 2018, and $1.0375 per share in 2017)		(89,637)					(89,637)
Treasury stock and other (in shares)			(243,000)
Treasury stock and other		(7,591)		(28)		(7,563)
Ending Balance (in shares) at Sep. 30, 2017			86,556,000
Ending Balance at Sep. 30, 2017		1,236,643	$ 222,258	219,696	(3,256)	(70,039)	867,984
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		233,436					233,436
Other comprehensive income (loss)		(6,078)			(6,078)
Common stock issued:
Incentive compensation plan (in shares)			561,000
Incentive compensation plan		16,572	$ 1,403	15,169
Dividend reinvestment plan (in shares)	[1]		413,000
Dividend reinvestment plan	[1]	17,094		755		16,339
Waiver discount (in shares)			1,014,000
Waiver discount		41,677	$ 2,535	39,142
Cash dividend declared ($1.19 per share in 2019, $1.11 per share in 2018, and $1.0375 per share in 2017)		(97,579)					(97,579)
Treasury stock and other (in shares)			(251,000)
Treasury stock and other		$ (22,787)		(14)		(22,773)
Reclassifications of certain income tax effects to retained earnings					(3,276)		3,276
Ending Balance (in shares) at Sep. 30, 2018		88,292,956	88,293,000
Ending Balance at Sep. 30, 2018		$ 1,418,978	$ 226,196	274,748	(12,610)	(76,473)	1,007,117
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		169,505					169,505
Other comprehensive income (loss)		(15,731)			(15,731)
Common stock issued:
Incentive compensation plan (in shares)			182,000
Incentive compensation plan		3,787	$ 453	3,334
Dividend reinvestment plan (in shares)	[1]		351,000
Dividend reinvestment plan	[1]	16,663		2,718		13,945
Waiver discount (in shares)			1,181,000
Waiver discount		57,391		10,531		46,860
Cash dividend declared ($1.19 per share in 2019, $1.11 per share in 2018, and $1.0375 per share in 2017)		(106,342)					(106,342)
Treasury stock and other (in shares)			(8,000)
Treasury stock and other		$ 5,232				5,232
Ending Balance (in shares) at Sep. 30, 2019		89,998,788	89,999,000
Ending Balance at Sep. 30, 2019		$ 1,551,717	$ 226,649	$ 291,331	$ (31,787)	$ (10,436)	$ 1,075,960

[1] Shares sold through the DRP are issued from treasury stock at average cost, which may differ from the actual market price paid.