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Investments in Unconsolidated Real Estate Ventures - Summary of Composition of Investments (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended	9 Months Ended
	Apr. 30, 2020	Jun. 30, 2020	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Schedule of Equity Method Investments [Line Items]
Total investments in unconsolidated real estate ventures			$ 463,026		$ 543,026
Distributions of capital from unconsolidated real estate ventures			$ 70,818	$ 7,557
Prudential Global Investment Management (PGIM)
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage			50.00%
Total investments in unconsolidated real estate ventures			$ 217,398		215,624
Landmark
Schedule of Equity Method Investments [Line Items]
Total investments in unconsolidated real estate ventures			$ 68,331		77,944
Landmark \| Minimum
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage			1.80%
Landmark \| Maximum
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage			49.00%
CBREI Venture
Schedule of Equity Method Investments [Line Items]
Total investments in unconsolidated real estate ventures			$ 65,598		68,405
CBREI Venture \| Minimum
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage			5.00%
CBREI Venture \| Maximum
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage			64.00%
Canadian Pension Plan Investment Board (CPPIB)
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage			55.00%
Total investments in unconsolidated real estate ventures			$ 48,041		109,911
Canadian Pension Plan Investment Board (CPPIB) \| 1900 N Street
Schedule of Equity Method Investments [Line Items]
Distributions of capital from unconsolidated real estate ventures		$ 70,800
Canadian Pension Plan Investment Board (CPPIB) \| 1900 N Street \| Mortgages Payable
Schedule of Equity Method Investments [Line Items]
Maximum borrowing capacity	$ 160,000
Proceeds from loan	134,500
Remaining borrowing capacity	$ 25,500
Berkshire Group
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage			50.00%
Total investments in unconsolidated real estate ventures			$ 49,329		46,391
Brandywine Realty Trust
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage			30.00%
Total investments in unconsolidated real estate ventures			$ 13,769		13,830
Pacific Life Insurance Company
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage			20.00%
Total investments in unconsolidated real estate ventures		0	$ 0		10,385
Pacific Life Insurance Company \| The Marriott Wardman Park hotel
Schedule of Equity Method Investments [Line Items]
Impairment charge		$ 6,500
Other
Schedule of Equity Method Investments [Line Items]
Total investments in unconsolidated real estate ventures			$ 560		$ 536