Real Estate Fund Investments - Summary of Ownership Interests in Alternative Investment Funds and Funds Underlying Investments (Parenthetical) (Details) $ in Thousands				12 Months Ended
	Nov. 24, 2015 USD ($) ft²	Sep. 30, 2015 USD ($) ft²	Sep. 01, 2015 USD ($)	Dec. 31, 2015 USD ($) ft²
Investment Holdings [Line Items]
Area of office properties | ft²				10,400,000
PGRESS and PGRESS-H
Investment Holdings [Line Items]
Redemption of preferred equity investment | $			$ 42,475
Realized gain on investment | $			$ 7,455
PGRESS Funds | Four Seven Zero Vanderbilt Avenue
Investment Holdings [Line Items]
Area of office properties | ft²				650,000
PGRESS Funds | 2 Herald Square [Member]
Investment Holdings [Line Items]
Area of office properties | ft²				369,000
Mezzanine Secured Loan | 26 Broadway [Member]
Investment Holdings [Line Items]
Area of office properties | ft²				836,000
Subordinate loan | $				$ 220,000
Subordinate to other debt				The loan has a fixed interest rate and is subordinate to $220,000 of other debt.
Mezzanine Secured Loan | 1440 Broadway [Member]
Investment Holdings [Line Items]
Area of office properties | ft²		751,546
Subordinate loan | $		$ 265,000
Subordinate to other debt				The loan bears interest at LIBOR plus 600 bps, and has a one-year extension option and is subordinate to $265,000 of other debt.
Loan extension term		1 year
Mezzanine loan | $		$ 40,000
Mezzanine Secured Loan | 700 Eighth Avenue [Member]
Investment Holdings [Line Items]
Area of office properties | ft²	26,126
Loan extension term	1 year
Mortgage and Mezzanine Loan | $	$ 80,000
Mezzanine Secured Loan | London Interbank Offered Rate L I B O R | 1440 Broadway [Member]
Investment Holdings [Line Items]
Basis spread on variable rate		6.00%
Mezzanine Secured Loan | London Interbank Offered Rate L I B O R | 700 Eighth Avenue [Member]
Investment Holdings [Line Items]
Basis spread on variable rate	6.00%