CONSOLIDATED STATEMENTS OF COMMON STOCK EQUITY - USD ($) $ in Thousands		Total	Cumulative Effect, Period of Adoption, Adjustment	Common Stock	Premium on Common Stock	Accumulated Other Comprehensive (Loss) Income	Accumulated Other Comprehensive (Loss) Income Cumulative Effect, Period of Adoption, Adjustment	Treasury Stock And Other	Retained Earnings	Retained Earnings Cumulative Effect, Period of Adoption, Adjustment
Beginning Balance (in shares) at Sep. 30, 2018				88,293,000
Beginning Balance at Sep. 30, 2018		$ 1,294,664		$ 226,196	$ 274,748	$ (12,610)		$ (76,473)	$ 882,803
Beginning Balance (Accounting Standards Update 2016-01) at Sep. 30, 2018			$ 0				$ (3,446)			$ 3,446
Beginning Balance (Accounting Standards Update 2017-05) at Sep. 30, 2018			4,970							4,970
Beginning Balance (Accounting Standards Update 2014-09) at Sep. 30, 2018			$ (2,736)							$ (2,736)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		123,935							123,935
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		(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				89,999,000
Ending Balance at Sep. 30, 2019		1,381,833		$ 226,649	291,331	(31,787)		(10,436)	906,076
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		163,007							163,007
Other comprehensive income (loss)		(12,528)				(12,528)
Common stock issued:
Common stock offering (in shares)				5,333,000
Common stock offering		212,900		$ 13,333	199,567
Incentive compensation plan (in shares)				105,000
Incentive compensation plan		3,772		$ 261	3,511
Dividend reinvestment plan (in shares)	[1]			520,000
Dividend reinvestment plan	[1]	18,157			2,833			15,324
Cash dividend declared		(121,582)							(121,582)
Treasury stock and other (in shares)				(8,000)
Treasury stock and other		$ (1,663)			(5,260)			3,597
Ending Balance (in shares) at Sep. 30, 2020		95,949,183		95,949,000
Ending Balance at Sep. 30, 2020		$ 1,643,896		$ 240,243	491,982	(44,315)		8,485	947,501
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		117,890							117,890
Other comprehensive income (loss)		9,787				9,787
Common stock issued:
Common stock offering		(2,823)			(2,823)
Incentive compensation plan (in shares)				84,000
Incentive compensation plan		4,263		$ 210	4,053
Dividend reinvestment plan (in shares)	[1]			431,000
Dividend reinvestment plan	[1]	15,156		$ 191	9,372			5,593
Cash dividend declared		(130,781)							(130,781)
Treasury stock and other (in shares)				(754,000)
Treasury stock and other		$ (26,526)						(26,526)
Ending Balance (in shares) at Sep. 30, 2021		95,709,662		95,710,000
Ending Balance at Sep. 30, 2021		$ 1,630,862		$ 240,644	$ 502,584	$ (34,528)		$ (12,448)	$ 934,610

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