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, 2019			89,999,000
Balance as of beginning of period at Sep. 30, 2019		$ 1,345,615	$ 226,649	$ 291,331	$ (31,787)	$ (10,436)	$ 869,858
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		75,752					75,752
Other comprehensive income		759			759
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)			96,000
Incentive compensation plan		3,292	$ 239	3,053
Dividend reinvestment plan (in shares)	[1]		80,000
Dividend reinvestment plan	[1]	3,499		314		3,185
Cash dividend declared		(29,846)					(29,846)
Treasury stock and other		(3,879)				(3,879)
Balance as of end of period (in shares) at Dec. 31, 2019			95,508,000
Balance as of end of period at Dec. 31, 2019		1,608,092	$ 240,221	494,265	(31,028)	(11,130)	915,764
Balance as of beginning of period (in shares) at Sep. 30, 2019			89,999,000
Balance as of beginning of period at Sep. 30, 2019		1,345,615	$ 226,649	291,331	(31,787)	(10,436)	869,858
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		149,598
Other comprehensive income		(2,852)
Balance as of end of period (in shares) at Mar. 31, 2020			95,643,000
Balance as of end of period at Mar. 31, 2020		1,654,737	$ 240,221	493,956	(34,639)	(4,523)	959,722
Balance as of beginning of period (in shares) at Dec. 31, 2019			95,508,000
Balance as of beginning of period at Dec. 31, 2019		1,608,092	$ 240,221	494,265	(31,028)	(11,130)	915,764
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		73,846					73,846
Other comprehensive income		(3,611)			(3,611)
Common stock issued:
Dividend reinvestment plan (in shares)	[1]		143,000
Dividend reinvestment plan	[1]	5,205		(416)		5,621
Cash dividend declared		(29,888)					(29,888)
Treasury stock and other (in shares)			(8,000)
Treasury stock and other		1,093		107		986
Balance as of end of period (in shares) at Mar. 31, 2020			95,643,000
Balance as of end of period at Mar. 31, 2020		$ 1,654,737	$ 240,221	493,956	(34,639)	(4,523)	959,722
Balance as of beginning of period (in shares) at Sep. 30, 2020		95,949,183	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		95,949,183	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		96,262,016	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:
Common stock offering		(388)		(388)
Incentive compensation plan (in shares)			28,000
Incentive compensation plan		1,216	$ 72	1,144
Dividend reinvestment plan (in shares)			103,000
Dividend reinvestment plan		3,579	$ 58	3,521
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		96,262,016	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

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