Investments in Unconsolidated Joint Ventures - Summary of Investments and Income from Investments In Unconsolidated Joint Ventures (Parenthetical) (Details) - USD ($) $ in Thousands		6 Months Ended	12 Months Ended
	May 05, 2017	Jun. 30, 2017	Dec. 31, 2017	Jun. 29, 2017	May 04, 2017	Dec. 31, 2016
Schedule Of Equity Method Investments [Line Items]
Basis of investment property			$ 44,762			$ 6,411
Distributions of capital from unconsolidated joint ventures			$ 20,000
RDF [Member]
Schedule Of Equity Method Investments [Line Items]
Ownership percentage in residential fund	7.40%		7.40%
712 Fifth Avenue [Member]
Schedule Of Equity Method Investments [Line Items]
Basis of investment property				$ 4,928		2,912
Distributions of capital from unconsolidated joint ventures		$ 20,000
Property ownership, percentage		50.00%
Difference between carrying amount of investment and equity			$ 20,336
60 Wall Street [Member]
Schedule Of Equity Method Investments [Line Items]
Basis of investment property			25,083
Difference between carrying amount of investment and equity			2,869
Oder-Center, Germany [Member]
Schedule Of Equity Method Investments [Line Items]
Basis of investment property			3,648			$ 3,499
Difference between carrying amount of investment and equity			5,166
75 Howard [Member]
Schedule Of Equity Method Investments [Line Items]
Basis of investment property			$ 16,031
75 Howard [Member] | RDF [Member]
Schedule Of Equity Method Investments [Line Items]
Property ownership, percentage					100.00%
Economic interest percentage	1.50%		1.50%