COMMON STOCK EQUITY - CHANGE IN COMMON STOCK EQUITY (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
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	123,699	$ 34,929
Other comprehensive income	(5,204)	$ 5,832
Common stock issued:
Incentive plan	$ 15,404
Dividend reinvestment plan (in shares)	90,000	139,000
Dividend reinvestment plan	$ 3,799
Waiver discount (in shares)	554,000
Waiver discount	$ 22,690
Cash dividend declared ($.2725 per share)	(23,831)
Treasury stock and other	$ (25,430)
Balance as of end of period (shares)	87,475,456
Balance as of end of period	$ 1,347,770
Cash dividend declared per share (usd per share)	$ 0.2725	$ 0.255
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)	525,000
Incentive plan	$ 1,453
Dividend reinvestment plan (in shares)	90,000
Waiver discount (in shares)	554,000
Waiver discount	$ 1,384
Treasury stock and other (shares)	(250,000)
Balance as of end of period (shares)	87,475,000
Balance as of end of period	$ 225,095
Premium on Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period	219,696
Common stock issued:
Incentive plan	13,951
Dividend reinvestment plan	245
Waiver discount	21,306
Treasury stock and other	(56)
Balance as of end of period	255,142
Accumulated Other Comprehensive (Loss) Income
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period	(3,256)	$ (15,155)
Other comprehensive income	(5,204)
Common stock issued:
Balance as of end of period	(8,460)	$ (9,323)
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	3,554
Treasury stock and other	(25,374)
Balance as of end of period	(91,859)
Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period	867,984
Net income	123,699
Common stock issued:
Cash dividend declared ($.2725 per share)	(23,831)
Balance as of end of period	$ 967,852