COMMON STOCK EQUITY - CHANGE IN COMMON STOCK EQUITY (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Dec. 31, 2015	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period (shares)				86,086,355
Balance as of beginning of period				$ 1,166,591
Net income		$ 18,957	$ (17,363)	168,588	$ 106,272
Other comprehensive income		$ (3,304)	$ (5,062)	4,254	$ 3,657
Common stock issued:
Incentive plan				$ 5,572
Dividend reinvestment plan (in shares)	5,000,000			381,000	368,000
Dividend reinvestment plan				$ 13,772
Cash dividend declared ($.765 per share)				(66,051)
Treasury stock and other				$ (8,125)
Balance as of end of period (shares)		86,465,874		86,465,874
Balance as of end of period		$ 1,284,601		$ 1,284,601
Cash dividend declared per share (usd per share)		$ 0.255	$ 0.24	$ 0.765	$ 0.72
Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period (shares)				86,086,000
Balance as of beginning of period				$ 221,654
Common stock issued:
Incentive plan (in shares)				238,000
Incentive plan				$ 595
Dividend reinvestment plan (in shares)				381,000
Treasury stock and other (shares)				(239,000)
Balance as of end of period (shares)		86,466,000		86,466,000
Balance as of end of period		$ 222,249		$ 222,249
Premium on Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period				215,580
Common stock issued:
Incentive plan				4,977
Dividend reinvestment plan				(1,224)
Treasury stock and other				(28)
Balance as of end of period		219,305		219,305
Accumulated Other Comprehensive (Loss) Income
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period		(7,597)	$ (675)	(15,155)	$ (9,394)
Other comprehensive income				4,254
Common stock issued:
Balance as of end of period		(10,901)	$ (5,737)	(10,901)	$ (5,737)
Treasury Stock And Other
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period				(81,044)
Common stock issued:
Dividend reinvestment plan				14,996
Treasury stock and other				(8,097)
Balance as of end of period		(74,145)		(74,145)
Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period				825,556
Net income				168,588
Common stock issued:
Cash dividend declared ($.765 per share)				(66,051)
Balance as of end of period		$ 928,093		$ 928,093