Investments in and Advances to Unconsolidated Affiliates - Fund Investments - Additional Information (Details)									9 Months Ended		12 Months Ended
	Aug. 21, 2019 USD ($) ft²	Jun. 29, 2019 USD ($)	Apr. 30, 2019 USD ($) ft²	Mar. 19, 2019 USD ($) ft²	Aug. 29, 2018 USD ($) property	Jun. 29, 2018 USD ($) property	May 15, 2018 USD ($)	Jan. 18, 2018 USD ($) property	Sep. 30, 2019 USD ($) property	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2014 property JointVenture	Oct. 31, 2018 segment
Schedule of Equity Method Investments [Line Items]
Purchase Price									$ 260,344,000		$ 186,463,000
Number of retail properties | property									182
Proceeds from the disposition of properties, net									$ 80,120,000	$ 52,759,000
Fund V Family Center at Riverdale
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage									89.42%
Tri City Plaza
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage									90.00%
Washington REIT Portfolio
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage									90.00%
BSP I and BSP II
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage												50.00%
Number of Joint Ventures | JointVenture												2
Broughton St. Portfolio Savannah, GA
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage									50.00%
Number of businesses acquired | property												23
Number of retail properties | property									13
Fund I Vs Broughton Street Portfolio Venture
Schedule of Equity Method Investments [Line Items]
Number of retail properties | property						2						8
Losses on cost method investments						$ 1,000,000.0
Fund I Vs Broughton Street Portfolio Venture | Partnership Interest | Maximum
Schedule of Equity Method Investments [Line Items]
Losses on cost method investments						$ 100,000
Fund IV | Disposed of by sale | Broughton St. Portfolio Savannah, GA
Schedule of Equity Method Investments [Line Items]
Number of retail properties | property					1			2				2
Proceeds from the disposition of properties, net					$ 2,100,000			$ 8,000,000.0
Gain (loss) on disposal					(300,000)			(400,000)
Fund IV | Disposed of by sale | Broughton St. Portfolio Savannah, GA | Partnership Interest
Schedule of Equity Method Investments [Line Items]
Gain (loss) on disposal								$ 0
Fund IV | Disposed of by sale | Broughton St. Portfolio Savannah, GA | Partnership Interest | Maximum
Schedule of Equity Method Investments [Line Items]
Gain (loss) on disposal					$ (100,000)
BSP I
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage													100.00%
BSP I | Consolidated Entity
Schedule of Equity Method Investments [Line Items]
Number of retail properties | segment													11
BSP II | Unconsolidated Affiliates
Schedule of Equity Method Investments [Line Items]
Number of retail properties | property									2
Fund III's Storage Post venture
Schedule of Equity Method Investments [Line Items]
Distribution from cost method investment		$ 1,600,000					$ 3,200,000
Cost method investments		0
Fund III's Storage Post venture | Partnership Interest
Schedule of Equity Method Investments [Line Items]
Distribution from cost method investment		$ 400,000					$ 800,000
Riverdale, Utah | Fund V Family Center at Riverdale
Schedule of Equity Method Investments [Line Items]
Joint venture interest percentage				99.35%
Undivided interest in property percentage				90.00%
Square footage of real estate property (in square feet) | ft²				428,000
Purchase Price				$ 48,500,000
Vernon, Connecticut | Tri City Plaza
Schedule of Equity Method Investments [Line Items]
Square footage of real estate property (in square feet) | ft²			300,000
Purchase Price			$ 36,700,000
Frederick County, Maryland | Washington REIT Portfolio | 225,000 Square Foot Property
Schedule of Equity Method Investments [Line Items]
Square footage of real estate property (in square feet) | ft²	225,000
Purchase Price	$ 21,800,000
Frederick County, Maryland | Washington REIT Portfolio | 300,00 Square Foot Property
Schedule of Equity Method Investments [Line Items]
Square footage of real estate property (in square feet) | ft²	300,000
Purchase Price	$ 33,100,000