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, 2018			88,293,000
Balance as of beginning of period at Sep. 30, 2018		$ 1,418,978	$ 226,196	$ 274,748	$ (12,610)	$ (76,473)	$ 1,007,117
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		86,248					86,248
Other comprehensive income		234			234
Common stock issued:
Incentive compensation plan (in shares)			137,000
Incentive compensation plan		2,134	$ 343	1,791
Dividend reinvestment plan (in shares)	[1]		82,000
Dividend reinvestment plan	[1]	3,692		454		3,238
Waiver discount (in shares)			168,000
Waiver discount		7,964		1,293		6,671
Cash dividend declared		(25,938)					(25,938)
Treasury stock and other		1,504				1,504
Balance as of end of period (in shares) at Dec. 31, 2018			88,680,000
Balance as of end of period at Dec. 31, 2018		1,497,050	$ 226,539	278,286	(15,822)	(65,060)	1,073,107
Balance as of beginning of period (in shares) at Sep. 30, 2018			88,293,000
Balance as of beginning of period at Sep. 30, 2018		1,418,978	$ 226,196	274,748	(12,610)	(76,473)	1,007,117
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		159,821
Other comprehensive income		539
Balance as of end of period (in shares) at Mar. 31, 2019			89,164,000
Balance as of end of period at Mar. 31, 2019		1,569,162	$ 226,613	283,429	(15,517)	(46,062)	1,120,699
Balance as of beginning of period (in shares) at Dec. 31, 2018			88,680,000
Balance as of beginning of period at Dec. 31, 2018		1,497,050	$ 226,539	278,286	(15,822)	(65,060)	1,073,107
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		73,573					73,573
Other comprehensive income		305			305
Common stock issued:
Incentive compensation plan (in shares)			30,000
Incentive compensation plan		1,224	$ 74	1,150
Dividend reinvestment plan (in shares)	[1]		123,000
Dividend reinvestment plan	[1]	5,762		870		4,892
Waiver discount (in shares)			339,000
Waiver discount		16,575		3,123		13,452
Cash dividend declared		(25,981)					(25,981)
Treasury stock and other (in shares)			(8,000)
Treasury stock and other		654				654
Balance as of end of period (in shares) at Mar. 31, 2019			89,164,000
Balance as of end of period at Mar. 31, 2019		$ 1,569,162	$ 226,613	283,429	(15,517)	(46,062)	1,120,699
Balance as of beginning of period (in shares) at Sep. 30, 2019		89,998,788	89,999,000
Balance as of beginning of period at Sep. 30, 2019		$ 1,551,717	$ 226,649	291,331	(31,787)	(10,436)	1,075,960
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		89,361					89,361
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		$ 1,827,803	$ 240,221	494,265	(31,028)	(11,130)	1,135,475
Balance as of beginning of period (in shares) at Sep. 30, 2019		89,998,788	89,999,000
Balance as of beginning of period at Sep. 30, 2019		$ 1,551,717	$ 226,649	291,331	(31,787)	(10,436)	1,075,960
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		177,866
Other comprehensive income		$ (2,852)
Balance as of end of period (in shares) at Mar. 31, 2020		95,643,185	95,643,000
Balance as of end of period at Mar. 31, 2020		$ 1,889,107	$ 240,221	493,956	(34,639)	(4,523)	1,194,092
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		1,827,803	$ 240,221	494,265	(31,028)	(11,130)	1,135,475
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		88,505					88,505
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		95,643,185	95,643,000
Balance as of end of period at Mar. 31, 2020		$ 1,889,107	$ 240,221	$ 493,956	$ (34,639)	$ (4,523)	$ 1,194,092

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