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
Balance as of beginning of period (in shares) at Sep. 30, 2020			95,949,000
Balance as of beginning of period 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)
Balance as of end of period (in shares) at Sep. 30, 2021			95,710,000
Balance as of end of period at Sep. 30, 2021		1,630,862	$ 240,644	502,584	(34,528)	(12,448)	934,610
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		274,922					274,922
Other comprehensive income (loss)		29,702			29,702
Common stock issued:
Incentive compensation plan (in shares)			193,000
Incentive compensation plan		9,146	$ 481	8,665
Dividend reinvestment plan (in shares)	[1]		355,000
Dividend reinvestment plan	[1]	14,741	$ 491	8,450		5,800
Cash dividend declared		(142,004)					(142,004)
Treasury stock and other (in shares)			(8,000)
Treasury stock and other		$ (159)		(2)		(157)
Balance as of end of period (in shares) at Sep. 30, 2022		96,249,859	96,250,000
Balance as of end of period at Sep. 30, 2022		$ 1,817,210	$ 241,616	519,697	(4,826)	(6,805)	1,067,528
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		264,724					264,724
Other comprehensive income (loss)		(5,133)			(5,133)
Common stock issued:
Incentive compensation plan (in shares)			136,000
Incentive compensation plan		5,168	$ 339	4,829
Dividend reinvestment plan (in shares)	[1]		258,000
Dividend reinvestment plan	[1]	15,034	$ 205	6,069		8,760
Waiver discount (in shares)			948,000
Waiver discount		42,807	$ 1,298	28,059		13,450
Cash dividend declared		(154,418)					(154,418)
Treasury stock and other (in shares)			(8,000)
Treasury stock and other		$ 5,343				5,343
Balance as of end of period (in shares) at Sep. 30, 2023		97,584,455	97,584,000
Balance as of end of period at Sep. 30, 2023		$ 1,990,735	$ 243,458	$ 558,654	$ (9,959)	$ 20,748	$ 1,177,834

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