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, 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		81,045					81,045
Other comprehensive income		1,043			1,043
Common stock issued:
Incentive compensation plan (in shares)			50,000
Incentive compensation plan		5,534	$ 124	5,410
Dividend reinvestment plan (in shares)	[1]		140,000
Dividend reinvestment plan	[1]	1,091		(4,502)		5,593
Cash dividend declared		(31,966)					(31,966)
Treasury stock and other		(2,429)				(2,429)
Balance as of end of period (in shares) at Dec. 31, 2020			96,139,000
Balance as of end of period at Dec. 31, 2020		1,698,214	$ 240,367	492,890	(43,272)	11,649	996,580
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		230,854
Other comprehensive income		2,119
Balance as of end of period (in shares) at Mar. 31, 2021			96,262,000
Balance as of end of period at Mar. 31, 2021		1,821,833	$ 240,497	497,167	(42,196)	11,983	1,114,382
Balance as of beginning of period (in shares) at Dec. 31, 2020			96,139,000
Balance as of beginning of period at Dec. 31, 2020		1,698,214	$ 240,367	492,890	(43,272)	11,649	996,580
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		149,809					149,809
Other comprehensive income		1,076			1,076
Common stock issued:
Incentive compensation plan (in shares)			28,000
Incentive compensation plan		1,216	$ 72	1,144
Dividend reinvestment plan (in shares)	[1]		103,000
Dividend reinvestment plan	[1]	3,579	$ 58	3,521
Common stock offering		(388)		(388)
Cash dividend declared		(32,007)					(32,007)
Treasury stock and other (in shares)			(8,000)
Treasury stock and other		334				334
Balance as of end of period (in shares) at Mar. 31, 2021			96,262,000
Balance as of end of period at Mar. 31, 2021		$ 1,821,833	$ 240,497	497,167	(42,196)	11,983	1,114,382
Balance as of beginning of period (in shares) at Sep. 30, 2021		95,709,662	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		95,709,662	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
Other comprehensive income		$ 2,061
Balance as of end of period (in shares) at Mar. 31, 2022		96,081,796	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		96,081,796	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

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