Investments in and Advances to Unconsolidated Affiliates - Core Portfolio - Additional Information (Details) $ in Thousands					1 Months Ended	9 Months Ended	12 Months Ended
	Mar. 28, 2018 USD ($)	Nov. 16, 2017 USD ($)	May 01, 2017 USD ($)	Jan. 04, 2017 USD ($) ft²	Apr. 29, 2016 USD ($)	Sep. 30, 2018 USD ($) ft² property	Dec. 31, 2017 USD ($)
Schedule of Equity Method Investments [Line Items]
Purchase Price						$ 105,916	$ 254,532
Notes receivable, net						109,410	153,829
Brandywine Portfolio
Schedule of Equity Method Investments [Line Items]
Preferred return						5,600
Core Portfolio
Schedule of Equity Method Investments [Line Items]
Purchase Price						1,337	42,800
Notes receivable, net						56,475	101,695
Note receivable accrued interest						200
Equity method investments						$ 233,271	216,583
Core Portfolio | Renaissance Portfolio
Schedule of Equity Method Investments [Line Items]
Percentage of voting interests acquired				20.00%
Square footage of real estate property (in square feet) | ft²						213,000
Number of businesses acquired | property						18
Equity method investment, ownership percentage						20.00%
Equity method investments						$ 32,495	$ 35,041
Core Portfolio | Brandywine Portfolio
Schedule of Equity Method Investments [Line Items]
Square footage of real estate property (in square feet) | ft²						1,000,000
Repayments of debt					$ 140,000
Note receivable exchanged	$ 22,000		$ 16,000
Notes receivable, net			153,400			$ 38,700
Note receivable accrued interest	$ 300		$ 300
Core Portfolio | Brandywine Portfolio | Exchange Transaction One
Schedule of Equity Method Investments [Line Items]
Note receivable exchanged		$ 16,000
Note receivable accrued interest		600
Core Portfolio | Brandywine Portfolio | Exchange Transaction Two
Schedule of Equity Method Investments [Line Items]
Note receivable exchanged		60,700
Note receivable accrued interest		$ 900
Core Portfolio | Market Square
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage by third party		38.89%	38.89%
Equity method investment, ownership percentage		22.22%	61.11%
Equity method investments			$ 16,300
Gain on equity method investment			$ 9,800
Core Portfolio | Town Center
Schedule of Equity Method Investments [Line Items]
Note receivable exchanged						22,000
Note receivable accrued interest						$ 300
Equity method investment, ownership percentage by third party	14.11%	38.89%				24.78%
Equity method investment, ownership percentage		22.22%				75.22%	61.11%
Equity method investments	$ 22,300	$ 61,600				$ 99,741	$ 78,801
Gain on equity method investment	$ 12,700	$ 34,500
Core Portfolio | Alexandria, Virginia | Renaissance Portfolio
Schedule of Equity Method Investments [Line Items]
Square footage of real estate property (in square feet) | ft²				6,200
Purchase Price				$ 3,000
Number of businesses acquired | property						2
Core Portfolio | Washington D.C. | Renaissance Portfolio
Schedule of Equity Method Investments [Line Items]
Number of businesses acquired | property						16