REAL ESTATE PROPERTIES (Details) (USD $)	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
REAL ESTATE PROPERTIES [Abstract]
Depreciation Expense During the Period	$ 13,079,000	$ 12,089,000
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
Real estate properties component balances [Abstract]
Land	237,851,000		235,394,000
Buildings and building improvements	1,069,005,000		1,056,783,000
Tenant and other improvements	262,603,000		258,267,000
Development	112,703,000		112,149,000
Real estate and development properties	1,682,162,000		1,662,593,000
Less accumulated depreciation	(464,873,000)		(451,805,000)
Real estate, net	$ 1,217,289,000		$ 1,210,788,000