INVESTMENTS IN REAL ESTATE (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
INVESTMENTS IN REAL ESTATE
Land	$ 343,564,000		$ 343,564,000		$ 363,612,000
Land improvements	26,177,000		26,177,000		26,747,000
Buildings and improvements	1,473,060,000		1,473,060,000		1,506,962,000
Furniture, fixtures, and equipment	4,458,000		4,458,000		9,720,000
Tenant improvements	159,793,000		159,793,000		146,205,000
Work in progress	7,891,000		7,891,000		8,126,000
Investments in real estate	2,014,943,000		2,014,943,000		2,061,372,000
Accumulated depreciation	(389,873,000)		(389,873,000)		(369,661,000)
Net investments in real estate	1,625,070,000		1,625,070,000		$ 1,691,711,000
Depreciation expense	$ 16,030,000	$ 15,063,000	$ 31,703,000	$ 31,343,000