COMMON STOCK EQUITY - CHANGE IN COMMON STOCK EQUITY (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended
	Dec. 31, 2015	Dec. 31, 2016	Dec. 31, 2015
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		34,929	$ 50,282
Other comprehensive income		5,832	$ 3,924
Common stock issued:
Incentive plan		$ 4,278
Dividend reinvestment plan (in shares)	5,000,000	139,000	127,000
Dividend reinvestment plan		$ 4,619
Cash dividend declared ($.255 per share)		(21,980)
Treasury stock and other		$ (8,908)
Balance as of end of period (shares)		86,196,239
Balance as of end of period		$ 1,185,361
Cash dividend declared per share (usd per share)		$ 0.255	$ 0.24
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)		200,000
Incentive plan		$ 499
Dividend reinvestment plan (in shares)		139,000
Treasury stock and other (shares)		(229,000)
Balance as of end of period (shares)		86,196,000
Balance as of end of period		$ 222,153
Premium on Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period		215,580
Common stock issued:
Incentive plan		3,779
Dividend reinvestment plan		(883)
Treasury stock and other		0
Balance as of end of period		218,476
Accumulated Other Comprehensive (Loss) Income
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period		(15,155)	$ (9,394)
Other comprehensive income		5,832	3,924
Common stock issued:
Balance as of end of period	$ (5,470)	(9,323)	$ (5,470)
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		5,502
Treasury stock and other		(8,908)
Balance as of end of period		(84,450)
Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of beginning of period		825,556
Net income		34,929
Common stock issued:
Cash dividend declared ($.255 per share)		(21,980)
Balance as of end of period		$ 838,505