Investments in and Advances to Unconsolidated Affiliates - Core Portfolio - Additional Information (Details) $ in Thousands					1 Months Ended	3 Months Ended	12 Months Ended
	Jan. 24, 2019 USD ($) ft²	Mar. 28, 2018 USD ($)	Nov. 16, 2017 USD ($)	May 01, 2017 USD ($)	Apr. 29, 2016 USD ($)	Mar. 31, 2019 USD ($) ft²	Dec. 31, 2018 USD ($)	Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]
Purchase Price						$ 32,194	$ 186,463
Notes receivable, net						109,769	109,613
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
Notes receivable, net						56,475	56,475
Note receivable accrued interest							200
Equity method investments						$ 230,886	231,432
Core Portfolio | Renaissance Portfolio
Schedule of Equity Method Investments [Line Items]
Percentage of voting interests acquired	20.00%
Equity method investment, ownership percentage						20.00%
Equity method investments						$ 33,059	32,458
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			$ 99,106	$ 99,758
Gain on equity method investment		$ 12,700	$ 34,500
Core Portfolio | Washington, D.C. | Renaissance Portfolio
Schedule of Equity Method Investments [Line Items]
Square footage of real estate property (in square feet) | ft²	7,300
Purchase Price	$ 10,700
Purchase price less the assumption of the outstanding mortgage	$ 4,700