Investments in and Advances to Unconsolidated Affiliates - Fund Investments (Details)				1 Months Ended	3 Months Ended
	Jan. 18, 2018 USD ($) property	Jun. 30, 2017 USD ($)	Feb. 15, 2017 USD ($)	Jan. 31, 2017 USD ($)	Mar. 31, 2018 USD ($) property	Dec. 21, 2017 USD ($) property	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Schedule of Equity Method Investments [Line Items]
Cost method investments					$ 556,000				$ 556,000
Equity in earnings (losses) of unconsolidated affiliates					1,684,000			$ 12,703,000
Mervyns I and Mervyns II
Schedule of Equity Method Investments [Line Items]
Distribution from equity method investment					1,100,000
Equity method investments					0
Albertson's
Schedule of Equity Method Investments [Line Items]
Distribution from cost method investment					2,400,000
Cost method investments					0
Equity in earnings (losses) of unconsolidated affiliates					2,000,000
Fund IV
Schedule of Equity Method Investments [Line Items]
Equity method investments					70,938,000				82,143,000
Fund III
Schedule of Equity Method Investments [Line Items]
Equity method investments					$ 367,000				$ 373,000
Broughton St. Portfolio Savannah, GA | Fund IV
Schedule of Equity Method Investments [Line Items]
Number of retail properties | property					16
Disposed of by sale
Schedule of Equity Method Investments [Line Items]
Gain (loss) on disposal							$ 3,300,000
Disposed of by sale | Partnership Interest
Schedule of Equity Method Investments [Line Items]
Gain (loss) on disposal							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	$ 400,000					$ 1,200,000
Number of retail properties | property	2					5
Proceeds from Sale of Real Estate Held-for-investment	$ 8,000,000
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	$ 0					$ 100,000