EQUITY - Narrative (Details) - USD ($)						12 Months Ended
	Jun. 30, 2015	Dec. 01, 2014	Oct. 03, 2014	May. 12, 2014	Mar. 21, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Class of Stock [Line Items]
Preferred stock, shares issued (in shares)					5,800,000	5,750,000	5,750,000
Preferred stock, dividend rate, percentage					7.125%
Redemption price per share (in dollars per share)					$ 25.00
Preferred stock, net proceeds from offering					$ 138,300,000	$ 0	$ 0	$ 138,249,000
Preferred stock, dividends in arrears						0
Convertible preferred stock, conversion price (in dollars per share)					$ 25.00
Convertible preferred stock, shares issued upon conversion (in shares)					1.7864
Common stock issuance (in shares)	5,900,000		6,900,000	8,100,000
Common stock, net proceeds from offering	$ 147,900,000		$ 160,600,000	$ 219,100,000		$ 139,403,000	$ 510,147,000	$ 34,517,000
Price per share (in dollars per share)	$ 25.06		$ 24.25	$ 28.35
Number of shares sold to underwriters upon exercise of option to purchase additional shares (in shares)			900,000	1,100,000
Common stock, par value (in dollars per share)		$ 0.01				$ 0.01	$ 0.01
Sales commissions, percent of gross proceeds		2.00%
Tax withholding obligations						5,000,000	8,100,000
2014 ATM Program [Member]
Class of Stock [Line Items]
Common stock issuance (in shares)						0
Aggregate gross proceeds (up to)		$ 200,000,000.0
Shares available for sale, amount						$ 76,500,000
2013 and 2014 ATM Program [Member]
Class of Stock [Line Items]
Common stock issuance (in shares)							5,000,000
Common stock, net proceeds from offering							$ 140,400,000
Average price per share of common stock sold under ATM Program							$ 28.18
Stock issuance costs, commissions, ATM Program							$ 2,800,000