REAL ESTATE PROPERTIES (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Real Estate Properties [Line Items]
Depreciation Expense During the Period	$ 14,945,000	$ 14,504,000	$ 44,503,000	$ 42,571,000
Real Estate Properties
Land	289,039,000		289,039,000		$ 283,116,000
Building and building improvements	1,329,723,000		1,329,723,000		1,284,961,000
Tenant and other improvements	341,409,000		341,409,000		326,896,000
Development	204,981,000		204,981,000		179,973,000
Real estate and development properties	2,165,152,000		2,165,152,000		2,074,946,000
Less accumulated depreciation	(642,142,000)		(642,142,000)		(600,526,000)
Real estate, net	$ 1,523,010,000		$ 1,523,010,000		$ 1,474,420,000
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