Debt - Summary of Outstanding Debt (Parenthetical) (Details) - USD ($)	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
50 Beale Street [Member]
Debt Instrument [Line Items]
Ownership interest rate of property	31.10%	31.10%
1633 Broadway [Member] | Mortgages and Notes Payable with Variable Rate [Member]
Debt Instrument [Line Items]
Additional Borrowing Capacity Subject To Performance Hurdles	$ 200,000,000
One Market Plaza [Member]
Debt Instrument [Line Items]
Ownership interest rate of property	49.00%	49.00%