Investment and Disposition Activities - Additional Information (Detail) - USD ($)																6 Months Ended	12 Months Ended
		Mar. 02, 2018	Feb. 13, 2018	Nov. 09, 2017	Aug. 14, 2017	Jul. 12, 2017	Jun. 29, 2017	Nov. 01, 2016	Oct. 05, 2016	Aug. 11, 2016	Jul. 29, 2016	May 19, 2016	May 12, 2016	Apr. 27, 2016	Apr. 13, 2016	Jun. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Feb. 20, 2018	Nov. 10, 2016	Feb. 29, 2016	Jan. 21, 2016	Dec. 31, 2015	Dec. 31, 2014
Investment And Disposition Activities [Line Items]
Loans receivable, net																	$ 8,400,000	$ 8,400,000					$ 5,280,000
Outstanding principal balance on loan receivable																	8,400,000	8,400,000					5,280,000	$ 24,005,000
Mortgage loans payable																	19,663,000	108,826,000
Mortgage loans, outstanding balance	[1]																8,400,000
Mortgage loans, outstanding balance																		203,466,000
Amount available to be drawn down																		326,993,000
Subsequent Event [Member]
Investment And Disposition Activities [Line Items]
Ownership percentage in real estate investment																			60.00%
Additional capital contributions		$ 1,667,000
Capital contributions		$ 136,003,000
Liquidation Value [Member]
Investment And Disposition Activities [Line Items]
Loans receivable, net																							2,756,000
EB5 Mezzanine Loan [Member]
Investment And Disposition Activities [Line Items]
Mortgage loans, outstanding balance																		$ 195,000,000
Mortgage Loans Payable [Member]
Investment And Disposition Activities [Line Items]
Mortgage loans payable																	$ 19,663,000
LIBOR floor rate																	1.56425%	0.77167%
Drawn down amount of loan																	$ 237,500,000
Mortgage loans, outstanding balance																		$ 240,041,000
Mezzanine [Member]
Investment And Disposition Activities [Line Items]
Drawn down amount of loan																	176,000,000
Mentor Retail LLC [Member]
Investment And Disposition Activities [Line Items]
Loans receivable, net																	$ 2,510,000						2,511,000
Lake Brandt, Greensboro, North Carolina [Member]
Investment And Disposition Activities [Line Items]
Proceeds from sale of real estate, gross													$ 6,296,000
Proceeds from sale of real estate, net													$ 20,000,000
Liquidation value of property																							20,000,000
446 High Line LLC [Member]
Investment And Disposition Activities [Line Items]
Capital contributions															$ 2,540,000
Loans receivable, net															3,175,000
Short term loan															$ 635,000
Interest rate on mortgage loans															12.00%
701 Seventh WRT Investor LLC [Member]
Investment And Disposition Activities [Line Items]
Mortgage loans payable								$ 510,000,000
Mezzanine loan payable								$ 255,500,000
LIBOR floor rate								0.40%
Maturity date								Nov. 09, 2018
701 Seventh WRT Investor LLC [Member] | Subsequent Event [Member]
Investment And Disposition Activities [Line Items]
Proceeds from sale of real estate, gross			$ 1,530,000
Ownership percentage in real estate investment			12.00%
Closing date of property			Apr. 30, 2018
701 Seventh WRT Investor LLC [Member] | Mortgage and Mezzanine Loans [Member]
Investment And Disposition Activities [Line Items]
Loans payable								$ 615,000,000
701 Seventh WRT Investor LLC [Member] | EB5 Mezzanine Loan [Member]
Investment And Disposition Activities [Line Items]
Interest Rate								5.90%
Mezzanine loan payable								$ 200,000,000
701 Seventh WRT Investor LLC [Member] | LIBOR [Member]
Investment And Disposition Activities [Line Items]
LIBOR plus rate								6.49%
701 Seventh WRT Investor LLC [Member] | LIBOR [Member] | Mortgage and Mezzanine Loans [Member]
Investment And Disposition Activities [Line Items]
LIBOR plus rate								8.00%
Stamford, CT [Member] | Highgrove [Member]
Investment And Disposition Activities [Line Items]
Proceeds from sale of real estate, gross												$ 87,500,000
Proceeds from sale of real estate, net												$ 77,767,000
Liquidation value of property																							85,000,000
Ownership percentage in real estate investment												83.70%
Buyer's deposit under the purchase contract																						$ 5,000,000
Amount of deposits refunded to prior potential purchaser												$ 1,500,000									$ 1,000,000
Jacksonville, Florida [Member]
Investment And Disposition Activities [Line Items]
Proceeds from sale of real estate, gross																$ 10,500,000
Liquidation value of property																							11,432,000
Loans receivable, net																$ 8,400,000
mortgage loans, ceiling interest rate																	8.00%
Contractual Maturity Date																Jul. 01, 2019
Jacksonville, Florida [Member] | LIBOR [Member]
Investment And Disposition Activities [Line Items]
Interest rate spread on mortgage loans																5.00%
mortgage loans, floor interest rate																6.00%
mortgage loans, ceiling interest rate																8.00%
Chicago, Illinois [Member] | One East Erie [Member]
Investment And Disposition Activities [Line Items]
Proceeds from sale of real estate, gross										$ 47,900,000
Proceeds from sale of real estate, net										$ 46,982,000
Liquidation value of property																							53,000,000
Chicago, Illinois [Member] | Mentor Retail LLC [Member]
Investment And Disposition Activities [Line Items]
Proceeds from sale of real estate, gross											$ 10,450,000
Liquidation value of property																							2,986,000
Aggregate distributions from venture																		3,906,000
Nominal % Ownership											49.90%
Chicago, Illinois [Member] | WRT One South State Lender LP and WRT Elad One South State Equity LP [Member]]
Investment And Disposition Activities [Line Items]
Gross proceeds from sale of equity investments														$ 95,270,000
Poipu Shopping Village [Member] | B Note [Member]
Investment And Disposition Activities [Line Items]
Loans receivable, net																				$ 2,741,000
Churchill [Member]
Investment And Disposition Activities [Line Items]
Loans receivable, net									$ 100,000														$ 0
Outstanding principal balance on loan receivable									333,000
Loan discounted payoff amount									$ 100,000
New York, NY [Member] | 701 Seventh WRT Investor LLC [Member]
Investment And Disposition Activities [Line Items]
Additional capital contributions																	$ 5,834,000	13,013,000
Capital contributions																	134,336,000	128,502,000
New York, NY [Member] | 701 Seventh WRT Investor LLC [Member] | Liquidating Trust [Member]
Investment And Disposition Activities [Line Items]
Capital contributions																	137,256,000
Houston Texas [Member] | Mosaic Apartments [Member]
Investment And Disposition Activities [Line Items]
Proceeds from sale of real estate, gross				$ 90,500,000
Proceeds from sale of real estate, net				$ 37,178,000
Liquidation value of property																		91,200,000
Ownership percentage in real estate investment				83.70%
Oklahoma [Memebr] | Summit Pointe Apartments [Member]
Investment And Disposition Activities [Line Items]
Proceeds from sale of real estate, gross					$ 17,550,000
Proceeds from sale of real estate, net					$ 5,824,000
Ownership percentage in real estate investment					80.00%
Lisle Illinois [Member] | 550 - 650 Corporetum [Member]
Investment And Disposition Activities [Line Items]
Proceeds from sale of real estate, gross						$ 9,300,000
Proceeds from sale of real estate, net						$ 7,920,000
Liquidation value of property																		8,940,000
Orlando Florida [Member]
Investment And Disposition Activities [Line Items]
Proceeds from sale of real estate, gross							$ 34,807,000
Proceeds from sale of real estate, net							$ 62,000
Liquidation value of property																		$ 35,053,000
Las Vegas Nevada [Member] | RE CDO [Member]
Investment And Disposition Activities [Line Items]
Aggregate distributions from venture																	$ 2,118,000
Las Vegas Nevada [Member] | RE CDO [Member] | Mortgage Loans Payable [Member]
Investment And Disposition Activities [Line Items]
Interest Rate																	5.52%

[1] (1) Carrying amount represents the estimated amount expected to be collected on disposition of the loan, plus contractual interest receivable at December 31, 2017.