Investments in and Advances to Unconsolidated Real Estate Ventures - Narrative (Details) - USD ($)	1 Months Ended	12 Months Ended
	Nov. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Equity Method Investments [Line Items]
Investment in unconsolidated real estate venture		$ 16,321,000	$ 23,027,000	$ 7,865,000
Capitol Point North
Schedule of Equity Method Investments [Line Items]
Ownership Interest		41.00%
Investment in unconsolidated real estate venture	$ 13,100,000
Mortgages payable
Schedule of Equity Method Investments [Line Items]
Proceeds from debt		$ 79,300,000
Repayment of mortgage loan		250,000,000
Ability to draw additional borrowing capacity		$ 143,700,000
Mortgages payable | Atlantic Plumbing
Schedule of Equity Method Investments [Line Items]
Debt instrument, face amount	$ 110,000,000
Term	5 years
Proceeds from debt	$ 100,000,000
Repayment of mortgage loan	88,400,000
Ability to draw additional borrowing capacity	$ 10,000,000
LIBOR | Mortgages payable | Atlantic Plumbing
Schedule of Equity Method Investments [Line Items]
Mortgage loan interest rate	1.50%