ISSUANCES OF EQUITY - Underwritten Public Offerings (Details) - USD ($) $ / shares in Units, $ in Thousands, shares in Millions					12 Months Ended
	Jul. 24, 2015	Dec. 16, 2014	Apr. 02, 2014	Jun. 21, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
ISSUANCES OF EQUITY
Issuance of stock on underwritten public offering (in shares)	7.0	4.0	11.5	6.3
Price of common stock (in dollars per share)	$ 25.50	$ 25.75	$ 12.75	$ 12.35	$ 9.23
Net proceeds received from the offerings	$ 174,100	$ 100,900	$ 137,700	$ 73,300	$ 174,140	$ 238,580	$ 73,296
Underwriting discount and other stock issuance costs	$ 4,400	$ 2,100	$ 8,900	$ 4,800