COMMON STOCK (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	12 Months Ended
	May 31, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning of Year (in shares)		70,965,622	68,334,860	65,430,084
New Issues During Year:
Dividend Reinvestment Plan (in shares)		417,095	2,604,424	2,881,556
Stock-Based Compensation Plan (in shares)		45,338	26,338	23,220
Public Equity Offering (in shares)	8,050,000	8,050,000	0	0
End of Year (in shares)		79,478,055	70,965,622	68,334,860
Par value of common stock (in USD per share)	$ 1.25	$ 1.25	$ 1.25
Net excess over par value recorded in premium on common stock		$ 207,500
Proceeds from issuance of common stock	$ 203,600	$ 214,426	$ 63,192	$ 80,680
Incremental shares included in diluted earnings per share (in shares)		112,590	195,139	150,062
Dividend per share (in USD per share)		$ 1.07	$ 1.02	$ 0.96
Equity capital raised through Dividend Reinvestment Plan		$ 10,800