REAL ESTATE PROPERTIES (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Real Estate Properties [Line Items]
Depreciation Expense During the Period	$ 14,504,000	$ 13,826,000	$ 42,571,000	$ 40,377,000
Real Estate Properties
Land	278,642,000		278,642,000		265,871,000
Building and building improvements	1,274,813,000		1,274,813,000		1,210,318,000
Tenant and other improvements	322,371,000		322,371,000		302,370,000
Development	169,623,000		169,623,000		148,767,000
Real estate and development properties	2,045,449,000		2,045,449,000		1,927,326,000
Less accumulated depreciation	(588,059,000)		(588,059,000)		(550,113,000)
Real estate, net	$ 1,457,390,000		$ 1,457,390,000		$ 1,377,213,000
Building [Member]
Real Estate Properties [Line Items]
Property, Plant and Equipment, Useful Life			40 years
Minimum [Member] | Improvements [Member]
Real Estate Properties [Line Items]
Property, Plant and Equipment, Useful Life			3 years
Maximum [Member] | Improvements [Member]
Real Estate Properties [Line Items]
Property, Plant and Equipment, Useful Life			15 years