SUBSEQUENT EVENTS - Equity Transactions (Details) - USD ($) $ / shares in Units, $ in Thousands			12 Months Ended
	Jan. 29, 2016	Jan. 01, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Subsequent events
Dividend paid			$ 57,602	$ 56,550	$ 47,594
Restricted Stock Awards | Performance-Based
Subsequent events
Vested (in shares)			184,666	45,551	0
9.25% Series A Preferred Stock
Subsequent events
Preferred stock, dividend rate (as a percent)			9.25%	9.25%
7.875% Series B Preferred Stock
Subsequent events
Preferred stock, dividend rate (as a percent)			7.875%	7.875%
7.125% Series C Preferred Stock
Subsequent events
Preferred stock, dividend rate (as a percent)			7.125%	7.125%
Common Stock
Subsequent events
Common stock redemption of common units (in shares)			268,947	438,631	4,414,950
Subsequent events
Subsequent events
Dividend paid		$ 100
Cash dividends declared, common stock (in dollars per share)	$ 0.1175
Subsequent events | Restricted Stock Awards | Performance-Based
Subsequent events
Vested (in shares)		113,903
Subsequent events | 9.25% Series A Preferred Stock
Subsequent events
Cash dividends declared, preferred stock (in dollars per share)	0.578125
Subsequent events | 7.875% Series B Preferred Stock
Subsequent events
Cash dividends declared, preferred stock (in dollars per share)	0.4921875
Subsequent events | 7.125% Series C Preferred Stock
Subsequent events
Cash dividends declared, preferred stock (in dollars per share)	$ 0.4453125
Subsequent events | Common Stock
Subsequent events
Common stock redemption of common units (in shares)		31,042