Investments in and Advances to Unconsolidated Affiliates - Fund Investments (Details) - USD ($)					1 Months Ended		3 Months Ended		9 Months Ended
	Jun. 30, 2017	Feb. 15, 2017	Dec. 21, 2016	Jan. 28, 2016	Jul. 31, 2017	Jan. 31, 2017	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Schedule of Equity Method Investments [Line Items]
Cost method investments							$ 556,000		$ 556,000		$ 957,000
Equity in earnings (losses) of unconsolidated affiliates							4,001,000	$ (102,000)	21,044,000	$ 3,592,000
Mervyns I and Mervyns II
Schedule of Equity Method Investments [Line Items]
Distribution from equity method investment					$ 1,000,000
Equity method investments							0		0
Albertson's
Schedule of Equity Method Investments [Line Items]
Distribution from cost method investment					$ 2,300,000
Cost method investments							0		0
Equity in earnings (losses) of unconsolidated affiliates							1,900,000
Cortlandt Towne Center
Schedule of Equity Method Investments [Line Items]
Consideration received			$ 57,800,000	$ 107,300,000
Proceeds from sale of equity method investments			25,200,000
Gain (loss) on disposal			$ 36,000,000	$ 65,400,000
Percentage of disposition			35.00%	65.00%
Cortlandt Towne Center | Mortgages
Schedule of Equity Method Investments [Line Items]
Repayments of debt			$ 32,600,000
Fund IV
Schedule of Equity Method Investments [Line Items]
Equity method investments							91,402,000		91,402,000		95,498,000
Fund III
Schedule of Equity Method Investments [Line Items]
Equity method investments							$ 409,000		$ 409,000		$ 8,349,000
Partnership Interest | Cortlandt Towne Center
Schedule of Equity Method Investments [Line Items]
Gain (loss) on disposal			$ 8,800,000
2819 Kennedy Boulevard | Fund IV
Schedule of Equity Method Investments [Line Items]
Consideration received						$ 19,000,000
Proceeds from sale of equity method investments						10,600,000
Gain (loss) on disposal						6,300,000
2819 Kennedy Boulevard | Fund IV | Mortgages
Schedule of Equity Method Investments [Line Items]
Repayments of debt						8,400,000
2819 Kennedy Boulevard | Affiliated Entity | Fund IV
Schedule of Equity Method Investments [Line Items]
Gain (loss) on disposal						6,200,000
2819 Kennedy Boulevard | Partnership Interest | Fund IV
Schedule of Equity Method Investments [Line Items]
Gain (loss) on disposal						$ 1,400,000
Arundel Plaza | Fund III
Schedule of Equity Method Investments [Line Items]
Consideration received		$ 28,800,000
Proceeds from sale of equity method investments		18,800,000
Gain (loss) on disposal		8,200,000
Arundel Plaza | Fund III | Mortgages
Schedule of Equity Method Investments [Line Items]
Repayments of debt		10,000,000
Arundel Plaza | Affiliated Entity | Fund III
Schedule of Equity Method Investments [Line Items]
Gain (loss) on disposal		5,300,000
Arundel Plaza | Partnership Interest | Fund III
Schedule of Equity Method Investments [Line Items]
Gain (loss) on disposal		$ 1,300,000
1701 Belmont Avenue | Fund IV
Schedule of Equity Method Investments [Line Items]
Consideration received	$ 5,600,000
Proceeds from sale of equity method investments	2,700,000
Gain (loss) on disposal	3,300,000
1701 Belmont Avenue | Fund IV | Mortgages
Schedule of Equity Method Investments [Line Items]
Repayments of debt	2,900,000
1701 Belmont Avenue | Affiliated Entity | Fund IV
Schedule of Equity Method Investments [Line Items]
Gain (loss) on disposal	3,300,000
1701 Belmont Avenue | Partnership Interest | Fund IV
Schedule of Equity Method Investments [Line Items]
Gain (loss) on disposal	$ 800,000