SUBSEQUENT EVENTS (Details)				12 Months Ended
	Feb. 15, 2018 USD ($) Property	Feb. 05, 2018 $ / shares shares	Jan. 01, 2018 USD ($) shares	Dec. 31, 2017 Property $ / shares shares	Dec. 31, 2016 shares	Jan. 24, 2018 contract
Subsequent events
Debt instrument, collateral, number of real estate properties | Property				50
7.125% Series C Preferred Stock
Subsequent events
Preferred stock, dividend rate (as a percent)				7.125%	7.125%
Preferred stock, shares outstanding | shares				3,400,000	3,400,000
Preferred stock, redemption price (in dollars per share)				$ 25
6.45% Series D Preferred Stock
Subsequent events
Preferred stock, dividend rate (as a percent)				6.45%	6.45%
Preferred stock, shares outstanding | shares				3,000,000	3,000,000
Preferred stock, redemption price (in dollars per share)				$ 25
6.25% Series E Preferred Stock
Subsequent events
Preferred stock, dividend rate (as a percent)				6.25%
Preferred stock, shares outstanding | shares				6,400,000
Preferred stock, redemption price (in dollars per share)				$ 25,000
Subsequent events
Subsequent events
Payments of dividends | $			$ 500,000
Number of stock award agreements in retirement separation agreement | contract						2
Cash dividends declared, common stock (in dollars per share)		$ 0.18
Subsequent events | Unsecured debt | 2018 Term Loan
Subsequent events
Loan amount | $	$ 225,000,000.0
Maximum increase in borrowing capacity available through accordion feature option | $	150,000,000.0
Amount drawn | $	$ 140,000,000
Debt instrument, collateral, number of real estate properties | Property	50
Subsequent events | Unsecured debt | 2018 Term Loan | Option One | LIBOR | Minimum
Subsequent events
Interest rate margin on variable rate basis	1.80%
Subsequent events | Unsecured debt | 2018 Term Loan | Option One | LIBOR | Maximum
Subsequent events
Interest rate margin on variable rate basis	2.55%
Subsequent events | Unsecured debt | 2018 Term Loan | Option Two | LIBOR
Subsequent events
Interest rate margin on variable rate basis	1.00%
Subsequent events | Unsecured debt | 2018 Term Loan | Option Two | Federal funds rate
Subsequent events
Interest rate margin on variable rate basis	0.50%
Subsequent events | Unsecured debt | 2018 Term Loan | Option Two | Base Rate | Minimum
Subsequent events
Interest rate margin on variable rate basis	0.80%
Subsequent events | Unsecured debt | 2018 Term Loan | Option Two | Base Rate | Maximum
Subsequent events
Interest rate margin on variable rate basis	1.55%
Subsequent events | 7.125% Series C Preferred Stock
Subsequent events
Cash dividends declared, preferred stock (in dollars per share)		$ 0.4453125
Preferred stock, shares outstanding | shares		3,400,000
Preferred stock, redemption price (in dollars per share)		$ 25.00
Subsequent events | 6.45% Series D Preferred Stock
Subsequent events
Cash dividends declared, preferred stock (in dollars per share)		0.403125
Subsequent events | 6.25% Series E Preferred Stock
Subsequent events
Cash dividends declared, preferred stock (in dollars per share)		$ 0.390625
Subsequent events | Performance-Based | Restricted Stock Awards | Executive officers
Subsequent events
Stock issued for awards vesting (in shares) | shares			309,010