Recent Transactions (Acquisitions) (Narrative) (Details) $ in Thousands						3 Months Ended
		Apr. 22, 2016 USD ($) ft²	Apr. 01, 2016 USD ($)	Jan. 19, 2016 USD ($)	Jan. 05, 2016 USD ($) item	Mar. 31, 2016 USD ($) item	Mar. 31, 2015 USD ($)
Real Estate Properties [Line Items]
Purchase price of property						$ 34,603	$ 19,658
Gain on change of control of interests						10,156
Mortgage loans, carrying amount						1,823,311
Realized gain on sale						58,600	$ 144
Costs of the project incurred						$ 10,600
Portside Apartment Holdings, L.L.C. [Member]
Real Estate Properties [Line Items]
Percentage of interest in venture						50.00%
PruRose Riverwalk G, L.L.C. [Member]
Real Estate Properties [Line Items]
Percentage of interest in venture	[1],[2],[3]					25.00%
Number of units | item	[2],[3]					316
Mortgage loans, carrying amount	[2],[3]					$ 79,392
Percentage of additional interest acquired						25.00%
Mortgage loan, maturity date	[2],[3]					Jul. 15, 2021
Portside Master Company, L.L.C. [Member]
Real Estate Properties [Line Items]
Percentage of interest in venture	[1],[2],[3]					38.25%
Number of units | item	[2],[3]					175
Mortgage loans, carrying amount	[2],[3]					$ 42,500
Spread over LIBOR	[4]					2.50%
Mortgage loan, maturity date	[2],[3]					Dec. 04, 2017
Subsequent Event [Member] | Portside Apartment Holdings, L.L.C. [Member]
Real Estate Properties [Line Items]
Percentage of interest in venture			85.00%
Purchase price of property			$ 38,100
Subsequent Event [Member] | PruRose Riverwalk G, L.L.C. [Member]
Real Estate Properties [Line Items]
Percentage of interest in venture			50.00%
Purchase price of property			$ 11,300
Subsequent Event [Member] | Portside Master Company, L.L.C. [Member]
Real Estate Properties [Line Items]
Percentage of interest in venture			85.00%
Mortgage loans, carrying amount			$ 42,500
Hoboken, New Jersey [Member] | Subsequent Event [Member]
Real Estate Properties [Line Items]
Area of property (in square feet) | ft²		566,000
Purchase price of property		$ 235,000
Overlook Ridge Apartments Investors, L.L.C. [Member]
Real Estate Properties [Line Items]
Percentage of interest in venture					50.00%
Number of units | item					371
Purchase price of property					$ 39,800
Loans assumed				$ 72,500	$ 52,700
Spread over LIBOR				3.625%
Mortgage loan, maturity date						Feb. 01, 2023
Multi-Family Properties [Member]
Real Estate Properties [Line Items]
Number of units | item						5,644

[1] Company's effective ownership % represents the Company's entitlement to residual distributions after payments of priority returns, where applicable.

[2] On April 1, 2016, the Company acquired the equity interests of its joint venture partner in Portside Apartment Holdings, L.L.C and PruRose Riverwalk G, L.L.C. for $38.1 million and $11.3 million, respectively, which increased its ownership to 85 percent in Portside Apartment Holdings, LLC and 50 percent in PruRose Riverwalk G, L.L.C. (See Note 3: Recent Transactions - Acquisitions).

[3] The Company's ownership interests in this venture are subordinate to its partner's preferred capital balance and the Company is not expected to meaningfully participate in the venture's cash flows in the near term.

[4] The construction loan has a maximum borrowing amount of $42,500 and provides, subject to certain conditions, two two-year extension options with a fee of 12.5 basis points for the first two-year extension and 25 basis points for the second two-year extension.