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, 2021			95,710,000
Balance as of beginning 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		111,312					111,312
Other comprehensive income		1,030			1,030
Common stock issued:
Incentive compensation plan (in shares)			147,000
Incentive compensation plan		7,502	$ 367	7,135
Dividend reinvestment plan (in shares)			105,000
Dividend reinvestment plan		3,678	$ 263	3,415
Cash dividend declared		(34,787)					(34,787)
Treasury stock and other		(2,619)				(2,619)
Balance as of end of period (in shares) at Dec. 31, 2021			95,962,000
Balance as of end of period at Dec. 31, 2021		1,716,978	$ 241,274	513,134	(33,498)	(15,067)	1,011,135
Balance as of beginning of period (in shares) at Sep. 30, 2021			95,710,000
Balance as of beginning 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		207,347
Balance as of end of period (in shares) at Mar. 31, 2022			96,082,000
Balance as of end of period at Mar. 31, 2022		1,784,287	$ 241,593	517,843	(32,467)	(15,025)	1,072,343
Balance as of beginning of period (in shares) at Dec. 31, 2021			95,962,000
Balance as of beginning of period at Dec. 31, 2021		1,716,978	$ 241,274	513,134	(33,498)	(15,067)	1,011,135
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		96,035					96,035
Other comprehensive income		1,031			1,031
Common stock issued:
Incentive compensation plan (in shares)			36,000
Incentive compensation plan		1,307	$ 91	1,216
Dividend reinvestment plan (in shares)			91,000
Dividend reinvestment plan		3,721	$ 228	3,493
Cash dividend declared		(34,827)					(34,827)
Treasury stock and other (in shares)			(7,000)
Treasury stock and other		42				42
Balance as of end of period (in shares) at Mar. 31, 2022			96,082,000
Balance as of end of period at Mar. 31, 2022		$ 1,784,287	$ 241,593	517,843	(32,467)	(15,025)	1,072,343
Balance as of beginning of period (in shares) at Sep. 30, 2022		96,249,859	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)			93,000
Dividend reinvestment plan		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		96,249,859	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
Balance as of end of period (in shares) at Mar. 31, 2023		96,901,373	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)	[1]		77,000
Dividend reinvestment plan	[1]	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		96,901,373	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

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