COMMON STOCK EQUITY - CHANGE IN COMMON STOCK EQUITY (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended		6 Months Ended
	Dec. 14, 2015	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 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		$ 73,328	$ 60,903	121,975	$ 184,223
Other comprehensive income		$ 4,795	$ (193)	8,719	$ 682
Common stock issued:
Incentive plan				$ 9,151
Dividend reinvestment plan (in shares)	5,000,000			260,000	279,000
Dividend reinvestment plan				$ 8,198
Cash dividend declared ($.48 per share)				(41,227)
Treasury stock and other				$ (6,290)
Balance as of end of period (shares)		85,968,887		85,968,887
Balance as of end of period		$ 1,207,482		$ 1,207,482
Cash dividend declared per share (usd per share)		$ 0.24	$ 0.23	$ 0.48	$ 0.45
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)				317,000
Incentive plan				$ 795
Dividend reinvestment plan (in shares)				260,000
Treasury stock and other (shares)				(139,000)
Balance as of end of period (shares)		85,969,000		85,969,000
Balance as of end of period		$ 221,633		$ 221,633
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				8,356
Dividend reinvestment plan				(2,271)
Treasury stock and other				(55)
Balance as of end of period		215,961		215,961
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				8,719	682
Common stock issued:
Balance as of end of period		(675)	$ (4,912)	(675)	$ (4,912)
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				10,469
Treasury stock and other				(6,235)
Balance as of end of period		(87,930)		(87,930)
Retained Earnings [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period				777,745
Net income				121,975
Common stock issued:
Cash dividend declared ($.48 per share)				(41,227)
Balance as of end of period		$ 858,493		$ 858,493