COMMON STOCK EQUITY - CHANGE IN COMMON STOCK EQUITY (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended
	Dec. 14, 2015	Dec. 31, 2015	Dec. 31, 2014
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		48,647	$ 123,320
Other comprehensive income		3,924	$ 875
Common stock issued:
Incentive plan		$ 5,499
Dividend reinvestment plan (in shares)	5,000,000	127,000	148,000
Dividend reinvestment plan		$ 3,805
Cash dividend declared ($.24 per share)		(20,594)
Treasury stock and other		$ (4,293)
Balance as of end of period (shares)		85,809,094
Balance as of end of period		$ 1,143,944
Cash dividend declared per share (usd per share)		$ 0.24	$ 0.225
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)		286,000
Incentive plan		$ 717
Dividend reinvestment plan (in shares)		127,000
Treasury stock and other (shares)		(135,000)
Balance as of end of period (shares)		85,809,000
Balance as of end of period		$ 221,555
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		4,782
Dividend reinvestment plan		(1,339)
Treasury stock and other		(23)
Balance as of end of period		213,351
Accumulated Other Comprehensive (Loss) Income [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period		(9,394)
Other comprehensive income		3,924
Common stock issued:
Balance as of end of period		(5,470)
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		5,144
Treasury stock and other		(4,270)
Balance as of end of period		(91,290)
Retained Earnings [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period		777,745
Net income		48,647
Common stock issued:
Cash dividend declared ($.24 per share)		(20,594)
Balance as of end of period		$ 805,798