REAL ESTATE PROPERTIES REAL ESTATE PROPERTIES 1 (Details) - USD ($)		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Land		$ 452,698,000	$ 380,684,000
Buildings and building improvements		1,907,963,000	1,732,592,000
Tenant and other mprovements		471,909,000	440,205,000
Right-of-Use Assets - Ground Leases (operating)	[1]	11,997,000	0
Development and value-add properties	[2]	419,999,000	263,664,000
Real estate, development and value-add properties		3,264,566,000	2,817,145,000
Real Estate Investment Property, Accumulated Depreciation		(871,139,000)	(814,915,000)
Real Estate Investment Property, Net		$ 2,393,427,000	$ 2,002,230,000
Percentage of occupancy to qualify as value-add property		75.00%
Percentage of acquisition price to be spent on capital items to qualify as value-add property		20.00%

[1] See Ground Leases discussion below and in Note 1(o) for information regarding the Company's right of use assets for ground leases.

[2] 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.