COMMON STOCK EQUITY - CHANGE IN COMMON STOCK EQUITY (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period (shares)			86,555,507
Balance as of beginning of period			$ 1,236,643
Net income	$ 140,266	$ 114,702	263,965	$ 149,631
Other comprehensive income	$ (5,328)	$ 1,726	(10,532)	$ 7,558
Common stock issued:
Incentive plan			$ 16,529
Dividend reinvestment plan (in shares)			242,000	279,000
Dividend reinvestment plan			$ 9,755
Waiver discount (in shares)			554,000
Waiver discount			$ 22,690
Cash dividend declared ($.545 per share)			(47,717)
Treasury stock and other			$ (23,959)
Balance as of end of period (shares)	87,656,004		87,656,004
Balance as of end of period	$ 1,467,374		$ 1,467,374
Cash dividend declared per share (usd per share)	$ 0.2725000	$ 0.255	$ 0.545	$ 0.51
Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period (shares)			86,556,000
Balance as of beginning of period			$ 222,258
Common stock issued:
Incentive plan (in shares)			555,000
Incentive plan			$ 1,531
Dividend reinvestment plan (in shares)			242,000
Waiver discount (in shares)			554,000
Waiver discount			$ 1,384
Treasury stock and other (shares)			(251,000)
Balance as of end of period (shares)	87,656,000		87,656,000
Balance as of end of period	$ 225,173		$ 225,173
Premium on Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period			219,696
Common stock issued:
Incentive plan			14,998
Dividend reinvestment plan			172
Waiver discount			21,306
Treasury stock and other			(14)
Balance as of end of period	256,158		256,158
Accumulated Other Comprehensive (Loss) Income
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period	(8,460)	$ (9,323)	(3,256)	$ (15,155)
Other comprehensive income			(10,532)
Common stock issued:
Balance as of end of period	(13,788)	$ (7,597)	(13,788)	$ (7,597)
Treasury Stock And Other
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period			(70,039)
Common stock issued:
Dividend reinvestment plan			9,583
Treasury stock and other			(23,945)
Balance as of end of period	(84,401)		(84,401)
Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period			867,984
Net income			263,965
Common stock issued:
Cash dividend declared ($.545 per share)			(47,717)
Balance as of end of period	$ 1,084,232		$ 1,084,232