INVESTMENTS IN REAL ESTATE (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
INVESTMENTS IN REAL ESTATE
Land	$ 364,966,000		$ 364,966,000		$ 354,035,000
Land improvements	28,181,000		28,181,000		28,071,000
Buildings and improvements	1,507,390,000		1,507,390,000		1,501,603,000
Furniture, fixtures, and equipment	9,087,000		9,087,000		10,875,000
Tenant improvements	142,500,000		142,500,000		131,446,000
Work in progress	6,380,000		6,380,000		10,764,000
Investments in real estate	2,058,504,000		2,058,504,000		2,036,794,000
Accumulated depreciation	(356,394,000)		(356,394,000)		(320,857,000)
Net investments in real estate	1,702,110,000		1,702,110,000		$ 1,715,937,000
Depreciation expense	$ 15,228,000	$ 14,969,000	$ 46,571,000	$ 43,821,000