COMMON STOCK EQUITY - CHANGE IN COMMON STOCK EQUITY (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended		9 Months Ended
	Dec. 14, 2015	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period (shares)				85,531,423
Balance as of beginning of period				$ 1,106,956
Net income		$ (17,363)	$ (7,460)	106,272	$ 176,763
Other comprehensive income		$ (5,062)	$ (318)	3,657	$ 364
Common stock issued:
Incentive plan				$ 9,261
Dividend reinvestment plan (in shares)	5,000,000			368,000	457,000
Dividend reinvestment plan				$ 12,086
Cash dividend declared ($.72 per share)				(61,885)
Treasury stock and other				$ (5,465)
Balance as of end of period (shares)		86,076,322		86,076,322
Balance as of end of period		$ 1,170,882		$ 1,170,882
Cash dividend declared per share (usd per share)		$ 0.24	$ 0.23	$ 0.72	$ 0.68
Common Stock [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period (shares)				85,531,000
Balance as of beginning of period				$ 220,838
Common stock issued:
Incentive plan (in shares)				321,000
Incentive plan				$ 804
Dividend reinvestment plan (in shares)				368,000
Treasury stock and other (shares)				(144,000)
Balance as of end of period (shares)		86,076,000		86,076,000
Balance as of end of period		$ 221,642		$ 221,642
Premium on Common Stock [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period				209,931
Common stock issued:
Incentive plan				8,457
Dividend reinvestment plan				(2,743)
Treasury stock and other				(55)
Balance as of end of period		215,590		215,590
Accumulated Other Comprehensive (Loss) Income [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period				(9,394)	$ (5,594)
Other comprehensive income				3,657	364
Common stock issued:
Balance as of end of period		(5,737)	$ (5,230)	(5,737)	$ (5,230)
Treasury Stock And Other [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period				(92,164)
Common stock issued:
Dividend reinvestment plan				14,829
Treasury stock and other				(5,410)
Balance as of end of period		(82,745)		(82,745)
Retained Earnings [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period				777,745
Net income				106,272
Common stock issued:
Cash dividend declared ($.72 per share)				(61,885)
Balance as of end of period		$ 822,132		$ 822,132