REAL ESTATE PROPERTIES (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Real Estate Investment Property, Net [Abstract]
Estimated useful lives for buildings (in years)	40
Estimated useful lives of improvements and personal property - minimum (in years)	3
Estimated useful lives of improvements and personal property - maximum (in years)	15
Depreciation Expense During the Period	$ 12,899,000	$ 12,014,000	$ 25,977,000	$ 24,103,000
Real Estate Properties
Land	237,907,000		237,907,000		235,394,000
Building and building improvements	1,068,376,000		1,068,376,000		1,056,783,000
Tenant and other improvements	266,033,000		266,033,000		258,267,000
Development	120,417,000		120,417,000		112,149,000
Real estate and development properties	1,692,733,000		1,692,733,000		1,662,593,000
Less accumulated depreciation	(475,389,000)		(475,389,000)		(451,805,000)
Real estate, net	$ 1,217,344,000		$ 1,217,344,000		$ 1,210,788,000