Investments in and Advances to Unconsolidated Affiliates - Fund Investments (Details)						1 Months Ended	3 Months Ended			6 Months Ended		12 Months Ended
	Jun. 29, 2018 USD ($) property	May 15, 2018 USD ($)	Jan. 18, 2018 USD ($) property	Jun. 30, 2017 USD ($)	Feb. 15, 2017 USD ($)	Jan. 31, 2017 USD ($)	Jun. 30, 2018 USD ($) property	Dec. 21, 2017 USD ($) property	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) property	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Schedule of Equity Method Investments [Line Items]
Cost method investments							$ 556,000			$ 556,000		$ 556,000
Equity in earnings (losses) of unconsolidated affiliates							$ 5,019,000		$ 4,340,000	6,703,000	$ 17,043,000
Proceeds from the disposition of properties, net										$ 25,218,000	0
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]
Cost method investment ownership interest							1.05%			1.05%
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							$ 71,242,000			$ 71,242,000		82,143,000
Fund IV's Broughton Street Portfolio venture
Schedule of Equity Method Investments [Line Items]
Number of retail properties | property	2
Losses on cost method investments	$ 1,000,000
Fund III
Schedule of Equity Method Investments [Line Items]
Equity method investments							$ 392,000			$ 392,000		$ 373,000
Fund III's Storage Post venture
Schedule of Equity Method Investments [Line Items]
Distribution from cost method investment		$ 3,200,000
Cost method investments		0
Partnership Interest | Fund IV's Broughton Street Portfolio venture
Schedule of Equity Method Investments [Line Items]
Losses on cost method investments	$ 100,000
Partnership Interest | Fund III's Storage Post venture
Schedule of Equity Method Investments [Line Items]
Distribution from cost method investment		$ 800,000
Broughton St. Portfolio Savannah, GA | Fund IV
Schedule of Equity Method Investments [Line Items]
Number of retail properties | property							14			14
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]
Gain (loss) on disposal						$ 6,300,000
Consideration received						19,000,000
Proceeds from sale of equity method investments						10,600,000
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]
Gain (loss) on disposal					$ 8,200,000
Consideration received					28,800,000
Proceeds from sale of equity method investments					18,800,000
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		$ 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]
Number of retail properties | property			2					5
Proceeds from the disposition of properties, net			$ 8,000,000
Gain (loss) on disposal			400,000					$ 1,200,000
Proceeds from sale of equity method investments								11,000,000
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