EQUITY - Narrative (Details) - USD ($)						9 Months Ended
	Jun. 30, 2015	Dec. 01, 2014	Oct. 03, 2014	May. 12, 2014	Mar. 21, 2013	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Equity [Line Items]
Preferred stock, shares issued (in shares)					5,800,000	5,750,000		5,750,000
Preferred stock, dividend rate, percentage					7.125%
Price per share (in dollars per share)	$ 25.06		$ 24.25	$ 28.35	$ 25.00
Preferred stock, net proceeds from offering					$ 138,300,000
Preferred stock, dividends in arrears						$ 0
Redemption share price (in dollars per share)					$ 25.00
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		$ 139,617,000	$ 229,825,000
Number of shares sold to underwriters upon exercise of option to purchase additional shares (in shares)			900,000	1,100,000
Tax withholding obligations on behalf of employees						4,700,000
Common stock, par value (in dollars per share)		$ 0.01				$ 0.01		$ 0.01
Sales commissions, percent of gross proceeds		2.00%
2014 ATM Program [Member]
Equity [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
Restricted Stock Units (RSUs) [Member]
Equity [Line Items]
Shares issued as a result of restricted stock unit vestings (in shares)						200,000