CONDENSED CONSOLIDATED STATEMENTS OF COMMON STOCK EQUITY (Unaudited) - 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, 2022			96,250,000
Balance as of beginning 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		115,921					115,921
Other comprehensive income		304			304
Common stock issued:
Incentive compensation plan (in shares)			92,000
Incentive compensation plan		3,472	$ 229	3,243
Dividend reinvestment plan (in shares)	[1]		93,000
Dividend reinvestment plan	[1]	3,866		437		3,429
Waiver discount (in shares)			368,000
Waiver discount		17,919		4,469		13,450
Cash dividend declared		(37,665)					(37,665)
Treasury stock and other		1,768				1,768
Balance as of end of period (in shares) at Dec. 31, 2022			96,803,000
Balance as of end of period at Dec. 31, 2022		1,922,795	$ 241,845	527,846	(4,522)	11,842	1,145,784
Balance as of beginning of period (in shares) at Sep. 30, 2022			96,250,000
Balance as of beginning 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		226,168
Other comprehensive income		609
Balance as of end of period (in shares) at Mar. 31, 2023			96,901,000
Balance as of end of period at Mar. 31, 2023		2,000,836	$ 241,919	529,819	(4,217)	15,075	1,218,240
Balance as of beginning of period (in shares) at Dec. 31, 2022			96,803,000
Balance as of beginning of period at Dec. 31, 2022		1,922,795	$ 241,845	527,846	(4,522)	11,842	1,145,784
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		110,247					110,247
Other comprehensive income		305			305
Common stock issued:
Incentive compensation plan (in shares)			29,000
Incentive compensation plan		1,170	$ 74	1,096
Dividend reinvestment plan (in shares)			77,000
Dividend reinvestment plan		3,671		877		2,794
Cash dividend declared		(37,791)					(37,791)
Treasury stock and other (in shares)			(8,000)
Treasury stock and other		439				439
Balance as of end of period (in shares) at Mar. 31, 2023			96,901,000
Balance as of end of period at Mar. 31, 2023		$ 2,000,836	$ 241,919	529,819	(4,217)	15,075	1,218,240
Balance as of beginning of period (in shares) at Sep. 30, 2023		97,584,455	97,584,000
Balance as of beginning of period at Sep. 30, 2023		$ 1,990,735	$ 243,458	558,654	(9,959)	20,748	1,177,834
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		89,411					89,411
Other comprehensive income		395			395
Common stock issued:
Incentive compensation plan (in shares)			116,000
Incentive compensation plan		3,741	$ 290	3,451
Dividend reinvestment plan (in shares)			94,000
Dividend reinvestment plan		3,788	$ 236	3,552
Waiver discount (in shares)			410,000
Waiver discount		17,919	$ 1,025	16,894
Cash dividend declared		(41,176)					(41,176)
Treasury stock and other (in shares)			(2,000)
Treasury stock and other		1,388				1,388
Balance as of end of period (in shares) at Dec. 31, 2023			98,202,000
Balance as of end of period at Dec. 31, 2023		$ 2,066,201	$ 245,009	582,551	(9,564)	22,136	1,226,069
Balance as of beginning of period (in shares) at Sep. 30, 2023		97,584,455	97,584,000
Balance as of beginning of period at Sep. 30, 2023		$ 1,990,735	$ 243,458	558,654	(9,959)	20,748	1,177,834
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		210,223
Other comprehensive income		$ 10,281
Balance as of end of period (in shares) at Mar. 31, 2024		98,744,915	98,745,000
Balance as of end of period at Mar. 31, 2024		$ 2,179,225	$ 246,367	604,009	322	22,936	1,305,591
Balance as of beginning of period (in shares) at Dec. 31, 2023			98,202,000
Balance as of beginning of period at Dec. 31, 2023		2,066,201	$ 245,009	582,551	(9,564)	22,136	1,226,069
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		120,812					120,812
Other comprehensive income		9,886			9,886
Common stock issued:
Incentive compensation plan (in shares)			25,000
Incentive compensation plan		1,242	$ 64	1,178
Dividend reinvestment plan (in shares)			84,000
Dividend reinvestment plan		3,655	$ 209	3,446		0
Waiver discount (in shares)			435,000
Waiver discount		17,919	$ 1,085	16,834
Cash dividend declared		(41,290)					(41,290)
Treasury stock and other (in shares)			(1,000)
Treasury stock and other		$ 800				800
Balance as of end of period (in shares) at Mar. 31, 2024		98,744,915	98,745,000
Balance as of end of period at Mar. 31, 2024		$ 2,179,225	$ 246,367	$ 604,009	$ 322	$ 22,936	$ 1,305,591

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