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 (shares) at Sep. 30, 2017				86,556,000
Balance as of beginning of period at Sep. 30, 2017		$ 1,236,643		$ 222,258	$ 219,696	$ (3,256)	$ (70,039)	$ 867,984
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		123,699						123,699
Other comprehensive income		(5,204)				(5,204)
Common stock issued:
Incentive compensation plan (in shares)				525,000
Incentive compensation plan		15,404		$ 1,453	13,951
Dividend reinvestment plan (in shares)	[1]			90,000
Dividend reinvestment plan	[1]	3,799			245		3,554
Waiver discount (in shares)				554,000
Waiver discount		22,690		$ 1,384	21,306
Cash dividend declared		(23,831)						(23,831)
Treasury stock and other (shares)				(250,000)
Treasury stock and other		25,430			56		25,374
Balance as of end of period (shares) at Dec. 31, 2017				87,475,000
Balance as of end of period at Dec. 31, 2017		1,347,770		$ 225,095	255,142	(8,460)	(91,859)	967,852
Balance as of beginning of period (shares) at Sep. 30, 2017				86,556,000
Balance as of beginning of period at Sep. 30, 2017		1,236,643		$ 222,258	219,696	(3,256)	(70,039)	867,984
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	[2]	249,691
Other comprehensive income		(12,624)
Balance as of end of period (shares) at Jun. 30, 2018				88,212,000
Balance as of end of period at Jun. 30, 2018		1,449,962		$ 226,189	274,138	(15,880)	(80,405)	1,045,920
Balance as of beginning of period (shares) at Dec. 31, 2017				87,475,000
Balance as of beginning of period at Dec. 31, 2017		1,347,770		$ 225,095	255,142	(8,460)	(91,859)	967,852
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		140,266						140,266
Other comprehensive income		(5,328)				(5,328)
Common stock issued:
Incentive compensation plan (in shares)				30,000
Incentive compensation plan		1,125		$ 78	1,047
Dividend reinvestment plan (in shares)	[1]			152,000
Dividend reinvestment plan	[1]	5,956			(73)		6,029
Cash dividend declared		(23,886)						(23,886)
Treasury stock and other (shares)				(1,000)
Treasury stock and other		1,471			42		1,429
Balance as of end of period (shares) at Mar. 31, 2018				87,656,000
Balance as of end of period at Mar. 31, 2018		1,467,374		$ 225,173	256,158	(13,788)	(84,401)	1,084,232
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		(14,274)	[2]					(14,274)
Other comprehensive income		(2,092)				(2,092)
Common stock issued:
Incentive compensation plan (in shares)				3,000
Incentive compensation plan		8		$ (135)	143
Dividend reinvestment plan (in shares)	[1]			92,000
Dividend reinvestment plan	[1]	3,642			1		3,641
Waiver discount (in shares)				460,000
Waiver discount		18,987		$ 1,151	17,836		0
Cash dividend declared		(24,038)						(24,038)
Treasury stock and other (shares)				1,000
Treasury stock and other		355					355
Balance as of end of period (shares) at Jun. 30, 2018				88,212,000
Balance as of end of period at Jun. 30, 2018		$ 1,449,962		$ 226,189	274,138	(15,880)	(80,405)	1,045,920
Balance as of beginning of period (shares) at Sep. 30, 2018		88,292,956		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 (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 (shares) at Sep. 30, 2018		88,292,956		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	[2]	151,419
Other comprehensive income		$ 844
Balance as of end of period (shares) at Jun. 30, 2019		89,917,854		89,918,000
Balance as of end of period at Jun. 30, 2019		$ 1,572,197		$ 226,627	290,414	(15,212)	(15,628)	1,085,996
Balance as of beginning of period (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 (shares)				(8,000)
Treasury stock and other		654					654
Balance as of end of period (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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		(8,402)	[2]					(8,402)
Other comprehensive income		305				305
Common stock issued:
Incentive compensation plan (in shares)				6,000
Incentive compensation plan		208		$ 14	194
Dividend reinvestment plan (in shares)	[1]			74,000
Dividend reinvestment plan	[1]	3,632			676		2,956
Waiver discount (in shares)				674,000
Waiver discount		32,852			6,115		26,737
Cash dividend declared		(26,301)						(26,301)
Treasury stock and other (shares)				0
Treasury stock and other		$ 741					741
Balance as of end of period (shares) at Jun. 30, 2019		89,917,854		89,918,000
Balance as of end of period at Jun. 30, 2019		$ 1,572,197		$ 226,627	$ 290,414	$ (15,212)	$ (15,628)	$ 1,085,996

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

[2] Includes income tax benefit related to the Tax Act of $844,000 and $57.7 million, for the three and nine months ended June 30, 2018, respectively.