Real Estate - Dispositions (Details) - USD ($) $ in Thousands	3 Months Ended				12 Months Ended
	Jun. 30, 2016	Mar. 31, 2016	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015
Fund III [Member] | Cortlandt Towne Center [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on Sale		$ 65,400
Fund III [Member] | Heritage Shops [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on Sale	$ 16,600
Fund III [Member] | Lincoln Park Centre [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on Sale				$ 27,100
Fund II [Member] | Liberty Avenue [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on Sale			$ 12,000
Fund II [Member] | City Point - Air Rights [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on Sale			$ 49,900
Disposal Group, Not Discontinued Operations [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales Price					$ 153,750	$ 203,878
Gain on Sale					81,965	89,063
Disposal Group, Not Discontinued Operations [Member] | Fund III [Member] | Cortlandt Towne Center [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales Price					107,250
Gain on Sale					65,393
Disposal Group, Not Discontinued Operations [Member] | Fund III [Member] | Heritage Shops [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales Price					46,500
Gain on Sale					$ 16,572
Disposal Group, Not Discontinued Operations [Member] | Fund III [Member] | Lincoln Park Centre [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales Price						64,000
Gain on Sale						27,143
Disposal Group, Not Discontinued Operations [Member] | Fund II [Member] | Liberty Avenue [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales Price						24,000
Gain on Sale						11,957
Disposal Group, Not Discontinued Operations [Member] | Fund II [Member] | City Point - Air Rights [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales Price						115,600
Gain on Sale						49,884
Disposal Group, Not Discontinued Operations [Member] | Fund I [Member] | Kroger Safeway Various [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales Price						278
Gain on Sale						$ 79