Subsequent Events (Details) - USD ($) $ in Thousands						1 Months Ended	3 Months Ended	6 Months Ended		12 Months Ended
	Jul. 27, 2017	Jul. 14, 2017	Jul. 07, 2017	Jul. 06, 2017	Jul. 05, 2017	Jul. 28, 2017	Sep. 30, 2017	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Subsequent Event [Line Items]
Proceeds from the disposition of properties								$ 0	$ 150,379
Purchase Price								79,712		$ 756,842
Cash distribution received								2,851	$ 2,974
Fund II
Subsequent Event [Line Items]
Derivative, notional amount								49,172
Mortgages | Interest Rate Swaps
Subsequent Event [Line Items]
Derivative, notional amount								67,300
Core Portfolio
Subsequent Event [Line Items]
Derivative, notional amount								$ 329,707
Purchase Price										$ 519,592
Subsequent Event
Subsequent Event [Line Items]
Purchase Price	$ 44,000
Subsequent Event | Albertson
Subsequent Event [Line Items]
Cash distribution received						$ 2,300
Subsequent Event | Interest Rate Swaps
Subsequent Event [Line Items]
Derivative term			10 years
Derivative, notional amount			$ 25,000
Subsequent Event | Core Portfolio | Mortgages
Subsequent Event [Line Items]
Repayments of debt					$ 28,600
New Hyde Park Shopping Center | Subsequent Event | Fund III | Disposed of by Sale
Subsequent Event [Line Items]
Proceeds from the disposition of properties				$ 22,100
City Point Tower I Property | Subsequent Event | Fund II | Disposed of by Sale
Subsequent Event [Line Items]
Proceeds from the disposition of properties		$ 96,000
Cost of real estate sales				$ 92,000
Forecast | City Point Tower I Property | Fund II | Disposed of by Sale
Subsequent Event [Line Items]
Loss on disposal of properties							$ 4,000