Investments in and Advances to Unconsolidated Affiliates - Fund Investments (Details)					1 Months Ended	3 Months Ended					12 Months Ended
	Jun. 30, 2017 USD ($)	Feb. 15, 2017 USD ($)	Dec. 21, 2016 USD ($)	Jan. 28, 2016 USD ($)	Jan. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) property properties	Dec. 21, 2017 USD ($) properties	Sep. 30, 2017 USD ($) property	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($) property	Dec. 31, 2017 USD ($) property properties	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Jan. 31, 2018 properties
Schedule of Equity Method Investments [Line Items]
Cost method investments						$ 556,000					$ 556,000	$ 957,000
Equity in earnings (losses) of unconsolidated affiliates											23,371,000	39,449,000	$ 37,330,000
Mervyns I and Mervyns II
Schedule of Equity Method Investments [Line Items]
Distribution from equity method investment											1,100,000
Equity method investments						0					0
Albertson's
Schedule of Equity Method Investments [Line Items]
Distribution from cost method investment											2,400,000
Cost method investments						0					0
Equity in earnings (losses) of unconsolidated affiliates											2,000,000
Fund IV
Schedule of Equity Method Investments [Line Items]
Equity method investments						82,143,000					82,143,000	95,498,000
Fund III
Schedule of Equity Method Investments [Line Items]
Equity method investments						$ 373,000					$ 373,000	$ 8,349,000
Broughton St. Portfolio Savannah, GA | Fund IV
Schedule of Equity Method Investments [Line Items]
Number of retail properties | properties						18					18
Disposed of by sale
Schedule of Equity Method Investments [Line Items]
Gain (loss) on disposal						$ 35,900,000		$ 13,000,000	$ 3,300,000	$ 14,500,000
Number of retail properties | property						3		2		2	3
Disposed of by sale | 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%
Disposed of by sale | Cortlandt Towne Center | Mortgages
Schedule of Equity Method Investments [Line Items]
Repayments of debt			$ 32,600,000
Disposed of by sale | Partnership Interest
Schedule of Equity Method Investments [Line Items]
Gain (loss) on disposal									800,000	$ 2,700,000
Disposed of by sale | Partnership Interest | Cortlandt Towne Center
Schedule of Equity Method Investments [Line Items]
Gain (loss) on disposal			$ 8,800,000
Disposed of by sale | 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
Disposed of by sale | 2819 Kennedy Boulevard | Fund IV | Mortgages
Schedule of Equity Method Investments [Line Items]
Repayments of debt					8,400,000
Disposed of by sale | 2819 Kennedy Boulevard | Affiliated Entity | Fund IV
Schedule of Equity Method Investments [Line Items]
Gain (loss) on disposal					6,200,000
Disposed of by sale | 2819 Kennedy Boulevard | Partnership Interest | Fund IV
Schedule of Equity Method Investments [Line Items]
Gain (loss) on disposal					$ 1,400,000
Disposed of by sale | 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
Disposed of by sale | Arundel Plaza | Fund III | Mortgages
Schedule of Equity Method Investments [Line Items]
Repayments of debt		10,000,000
Disposed of by sale | Arundel Plaza | Affiliated Entity | Fund III
Schedule of Equity Method Investments [Line Items]
Gain (loss) on disposal		5,300,000
Disposed of by sale | Arundel Plaza | Partnership Interest | Fund III
Schedule of Equity Method Investments [Line Items]
Gain (loss) on disposal		$ 1,300,000
Disposed of by sale | 1701 Belmont Avenue | Fund IV
Schedule of Equity Method Investments [Line Items]
Consideration received	$ 5,600,000								$ 5,600,000
Proceeds from sale of equity method investments	2,700,000
Disposed of by sale | 1701 Belmont Avenue | Fund IV | Mortgages
Schedule of Equity Method Investments [Line Items]
Repayments of debt	2,900,000
Disposed of by sale | 1701 Belmont Avenue | Affiliated Entity | Fund IV
Schedule of Equity Method Investments [Line Items]
Gain (loss) on disposal	$ 3,300,000
Disposed of by sale | Broughton St. Portfolio Savannah, GA | Fund IV
Schedule of Equity Method Investments [Line Items]
Proceeds from sale of equity method investments							$ 11,000,000
Gain (loss) on disposal							$ 1,200,000
Number of retail properties | properties							5
Disposed of by sale | Broughton St. Portfolio Savannah, GA | Affiliated Entity | Fund IV
Schedule of Equity Method Investments [Line Items]
Gain (loss) on disposal							$ 600,000
Disposed of by sale | Broughton St. Portfolio Savannah, GA | Partnership Interest | Fund IV
Schedule of Equity Method Investments [Line Items]
Gain (loss) on disposal							$ 100,000
Subsequent Event | Disposed of by sale | Broughton St. Portfolio Savannah, GA | Fund IV
Schedule of Equity Method Investments [Line Items]
Number of retail properties | properties														2