Real Estate Assets, Discontinued Operations and Assets Held for Sale (Aggregate Balance Sheet of Properties Held for Sale Included in Discontinued Operations) (Details) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2021 USD ($) buildings entities	Dec. 31, 2020 USD ($) buildings	Sep. 21, 2021 buildings	Sep. 30, 2020 entities
Held for Sale Assets in Continuing and Discontinued Operations
Continuing Operations, Number of In-service Properties Held for Sale | buildings	4
Continuing Operations, Number of Real Estate Properties Sold | buildings	29	7
Total Properties Sold or Classified as Held for Sale | buildings	40
Number of Real Estate Properties | entities	4			5
Equity Method Investment, Ownership Percentage	20.00%
Total assets held-for-sale	$ 198,914	$ 67,946
Total liabilities related to assets held-for-sale	$ 12,243	7,740
Disposal Group, Held-for-sale, Not Discontinued Operations
Held for Sale Assets in Continuing and Discontinued Operations
Number of Real Estate Properties | buildings	4
Continuing Operations [Member]
Held for Sale Assets in Continuing and Discontinued Operations
Land and improvements	$ 72,685	27,954
Buildings and tenant improvements	146,042	44,800
Accumulated depreciation	(37,128)	(5,976)
Deferred leasing and other costs, net	6,343	936
Other assets	10,972	232
Total assets held-for-sale	198,914	67,946
Accrued expenses	596	660
Other liabilities	11,647	7,080
Total liabilities related to assets held-for-sale	$ 12,243	$ 7,740
Tranche 3 | Disposal Group, Held-for-sale, Not Discontinued Operations
Held for Sale Assets in Continuing and Discontinued Operations
Number of Real Estate Properties | buildings			3