REAL ESTATE PROPERTIES REAL ESTATE PROPERTIES 1 (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Percentage of occupancy to qualify as value-add property		75.00%
portion of acquisition cost spent to qualify as value-add		20.00%
Land		$ 380,684	$ 345,424
Investment Building and Building Improvements		1,732,592	1,587,130
Tenant Improvements		440,205	404,180
Development and value-add properties	[1]	263,664	242,014
Real estate, development and value-add properties		2,817,145	2,578,748
Real Estate Investment Property, Accumulated Depreciation		(814,915)	(749,601)
Real Estate Investment Property, Net		$ 2,002,230	$ 1,829,147

[1] Value-add properties are defined as properties that are either acquired but not stabilized or can be converted to a higher and better use. Acquired properties meeting either of the following two conditions are considered value-add properties: (1) Less than 75% occupied as of the acquisition date (or will be less than 75% occupied within one year of acquisition date based on near term lease roll), or (2) 20% or greater of the acquisition cost will be spent to redevelop the property.