Agreements and Transactions with Related Parties, Other (Details) $ in Thousands	1 Months Ended							3 Months Ended			12 Months Ended
	Apr. 30, 2018 USD ($)	Dec. 31, 2017 USD ($) property	Aug. 31, 2017 USD ($)	Jul. 31, 2017 USD ($)	May 31, 2017 USD ($)	Mar. 31, 2017 USD ($) item	Jan. 31, 2017 USD ($)	Mar. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($) property item	Mar. 31, 2017 USD ($) item	Dec. 31, 2017 USD ($) property shares	Dec. 31, 2016 USD ($) property shares	Dec. 31, 2015 USD ($) loan	Dec. 31, 2014 USD ($) loan item	Jan. 31, 2018 USD ($)	Mar. 31, 2016 USD ($)
Agreements and transactions with related parties
Due from related party		$ 688						$ 3,719	$ 688		$ 688
Due to related party								1,558
Related party financing		50,000							50,000		50,000
Outstanding principal balance of related party financing		50,000							50,000		50,000
Income from equity affiliates								746		$ 763
Investments in equity affiliates		23,653						23,625	23,653		23,653
Residential Mortgage Banking Company
Agreements and transactions with related parties
Maximum percentage of guaranty provided by the Company in relation to the settlement													50.00%
Maximum exposure under this guaranty													$ 3,100
Preferred equity investments
Agreements and transactions with related parties
Principal amount		5,000					$ 15,000		5,000		5,000
Commitment to fund additional investment							$ 5,000
Interest income recorded										200
Fixed rate of interest (as a percent)							11.00%
Amount of ownership interest of related party in the entity							$ 5,000
Mezzanine loans
Agreements and transactions with related parties
Interest income recorded								100		100
Fixed rate of interest (as a percent)													12.50%
Amount of loan to related party													$ 3,000
ACM / Our "Former Manager"
Agreements and transactions with related parties
Due from related party		700						3,700	700		$ 700
Due to related party								1,600
Related party financing															$ 50,000
Interest expense on related party loan								300		1,000
Outstanding principal balance of related party financing															$ 50,000
ACM / Our "Former Manager" | Residential Mortgage Banking Company
Agreements and transactions with related parties
Noncontrolling interest in equity method investment acquired (as a percent)													50.00%
Indirect ownership percentage													22.5%
Acquisition purchase price													$ 9,600
Income from equity affiliates								$ 100		100
Maximum percentage of guaranty provided by the Company in relation to the settlement													50.00%
ACM / Our "Former Manager" | ACM Acquisition
Agreements and transactions with related parties
Number of preferred stock shares paired with each OP units | shares												1
Number of shares held by related party | shares								5,349,053
OP units hold as part of acquisition | shares								15,450,421			5,780,348	21,230,769
Percentage of voting power held by related party												24.80%
ACM / Our "Former Manager" | Non-qualified Residential Mortgages | Residential Mortgage Banking Company
Agreements and transactions with related parties
Additional investment made by the company along with a consortium of independent outside investors													$ 16,100
Distributions received								$ 200					16,400
ACM / Our "Former Manager" | Mortgage loans
Agreements and transactions with related parties
Amount of mortgage loan secured by property, purchased by related party													$ 47,000
ACM / Our "Former Manager" | Preferred equity interest financing agreement
Agreements and transactions with related parties
Related party financing		50,000							50,000		$ 50,000
Interest expense on related party loan								300		1,000
Outstanding principal balance of related party financing		50,000							50,000		50,000
ACM / Our "Former Manager" | Maturity date of April 2016 | Mortgage loans
Agreements and transactions with related parties
Base spread (as a percent)														4.80%
Interest income recorded								200		200
Number of mortgage loans secured by property purchased from related party | loan														2
ACM / Our "Former Manager" | Maturity date of April 2016 | Mortgage loans | First mortgage
Agreements and transactions with related parties
Amount of mortgage loan secured by property, purchased by related party														$ 14,600
ACM / Our "Former Manager" | Maturity date of April 2016 | Mortgage loans | Second mortgage
Agreements and transactions with related parties
Proceeds from repayment in full												$ 5,100
Amount of mortgage loan secured by property, purchased by related party														$ 5,100
ACM / Our "Former Manager" | Maturity date April 2018, extended from April 2016 | Mortgage loans | First mortgage
Agreements and transactions with related parties
Amount of mortgage loan secured by property, purchased by related party																$ 14,600
Consortium of investors including an immediate family member of our officers | Fannie Mae loan on a multifamily property
Agreements and transactions with related parties
Percentage of maximum loss-sharing obligation unpaid principal balance					5.00%
Principal amount					$ 46,900
Percentage of ownership interest of related party in the entity					21.40%
Consortium of investors including an immediate family member of our officers | Fannie Mae loan on a multifamily property | Maximum
Agreements and transactions with related parties
Servicing revenue								100
Consortium of Investors, including our Chief Executive Officer and our Former Manager
Agreements and transactions with related parties
Amount invested						$ 2,000				$ 2,000
Ownership interest (as a percent)						26.10%				26.10%
Number of portfolios of multifamily properties | item						2				2
Certain officers, including our Chief Executive Officer and our Former Manager | Maturity Date of January 2021 | Bridge loan, several multifamily properties
Agreements and transactions with related parties
Due to related party	$ 9,400
Percentage of ownership interest of related party in the entity	75.00%
Base spread (as a percent)	5.00%
LIBOR floor (as a percentage)	1.25%
Certain officers, including our Chief Executive Officer and our Former Manager | Maturity date of September 2019 | Bridge loan, six multifamily properties
Agreements and transactions with related parties
Principal amount												$ 48,000
Number of properties owned | property												6
Base spread (as a percent)												4.50%
LIBOR floor (as a percentage)												0.25%
Interest income recorded								600		$ 700
Proceeds from repayment in full			$ 6,800
Certain officers, including our Chief Executive Officer and our Former Manager | Maturity date of September 2019 | Bridge loan, six multifamily properties | Minimum
Agreements and transactions with related parties
Percentage of ownership interest of related party in the entity												10.50%
Certain officers, including our Chief Executive Officer and our Former Manager | Maturity date of September 2019 | Bridge loan, six multifamily properties | Maximum
Agreements and transactions with related parties
Percentage of ownership interest of related party in the entity												12.00%
Certain officers, including our Chief Executive Officer and our Former Manager | Maturity Date of October 2018 | Bridge loans
Agreements and transactions with related parties
Base spread (as a percent)													5.00%
LIBOR floor (as a percentage)													0.25%
Interest income recorded								300		300
Number of mortgage loans secured by property purchased from related party | loan													2
Loan due from related party													$ 16,700
Certain Officers | Maturity date of June 2020 | Bridge loan, several multifamily properties
Agreements and transactions with related parties
Due to related party		$ 32,800							$ 32,800		$ 32,800
Percentage of ownership after transaction		90.00%
Base spread (as a percent)		5.00%							5.00%		5.00%
LIBOR floor (as a percentage)		1.13%
Interest income recorded								500
Certain Officers | Maturity date of fourth quarter 2020 | Bridge loan, two multifamily properties
Agreements and transactions with related parties
Principal amount		$ 28,000							$ 28,000		$ 28,000
Number of properties owned | property		2							2		2
Percentage of ownership interest of related party in the entity									45.00%
Base spread (as a percent)		5.25%							5.25%		5.25%
Interest income recorded								500
Number of bridge loans originated | item									2
Certain Officers | Maturity date of fourth quarter 2020 | Bridge loan, two multifamily properties | Minimum
Agreements and transactions with related parties
LIBOR floor (as a percentage)									1.24%
Certain Officers | Maturity date of fourth quarter 2020 | Bridge loan, two multifamily properties | Maximum
Agreements and transactions with related parties
LIBOR floor (as a percentage)									1.54%
Certain Officers | Maturity date of July 2020 | Bridge loan, one multifamily property
Agreements and transactions with related parties
Principal amount				$ 36,000
Percentage of ownership interest of related party in the entity				95.00%
Base spread (as a percent)				4.50%
LIBOR floor (as a percentage)				1.00%
Interest income recorded								600
Certain Officers | Maturity date of January 2019 | Bridge loan, two multifamily properties
Agreements and transactions with related parties
Principal amount												$ 12,700
Number of properties owned | property												2
Percentage of ownership interest of related party in the entity												50.00%
Base spread (as a percent)												4.50%
LIBOR floor (as a percentage)												0.25%
Interest income recorded								300		300
Certain Officers | Maturity date of January 2019 | Bridge loan, one multifamily property
Agreements and transactions with related parties
Principal amount												$ 19,000
Percentage of ownership interest of related party in the entity												7.50%
Base spread (as a percent)												4.50%
LIBOR floor (as a percentage)												0.25%
Interest income recorded								$ 300		300
Certain Officers | Maturity date May 2018, extended from November 2017 | Preferred equity investments | Multifamily
Agreements and transactions with related parties
Equity investment												$ 5,200
Fixed rate of interest (as a percent)												10.00%
Certain Officers | Maturity date of November 2018 | Bridge loans
Agreements and transactions with related parties
Percentage of ownership interest of related party in the entity													7.50%
Base spread (as a percent)													4.50%
LIBOR floor (as a percentage)													0.25%
Interest income recorded										100
Amount of loan to related party													$ 7,100
Chief executive officer | Minimum
Agreements and transactions with related parties
Ownership interest allowed under company charter (as a percent)								5.00%
Chief executive officer | Bridge loan, two multifamily properties
Agreements and transactions with related parties
Number of properties owned | item														2
Percentage of ownership interest of related party in the entity														95.00%
Base spread (as a percent)														5.50%
Interest income recorded										100
Amount of loan to related party														$ 1,700
Investments in equity affiliates														$ 100
Ownership interest (as a percent)														5.00%
Chief executive officer | Maturity date of August 2017 | Bridge loans
Agreements and transactions with related parties
Percentage of ownership interest of related party in the entity														24.00%
Amount of mortgage loan secured by property, purchased by related party														$ 30,400
Chief executive officer | Maturity date of August 2020 | Bridge loans
Agreements and transactions with related parties
Base spread (as a percent)			4.00%
LIBOR floor (as a percentage)			1.23%
Amount of mortgage loan secured by property, purchased by related party			$ 43,200
Mr. Ivan Kaufman and his affiliates | Maturity date of August 2017 | Mezzanine loans
Agreements and transactions with related parties
Interest income recorded										800
Fixed rate of interest (as a percent)												12.00%
Amount of mortgage loan secured by property, purchased by related party												$ 4,600
Kaufman Entities
Agreements and transactions with related parties
Percentage of our Former Manager's outstanding membership interest of related party in another related party								75.00%
Affiliate of Mr. Ivan Kaufman
Agreements and transactions with related parties
Contract period with the new entity								7 years 6 months
Fees as a percentage of gross revenues of the underlying properties								4.75%
Lexford Portfolio
Agreements and transactions with related parties
Aggregate outstanding balance of related party debt								$ 310,300
Lexford Portfolio | Preferred equity investments
Agreements and transactions with related parties
Income from equity affiliates								$ 600		700
Ginkgo | Fannie Mae
Agreements and transactions with related parties
Loan purchased a multifamily apartment complex which assumed							$ 8,300
Percentage of maximum loss-sharing obligation unpaid principal balance							20.00%
Percentage of loan assumption fee							1.00%
Percentage of ownership after transaction								3.60%
Ginkgo | Maximum | Fannie Mae
Agreements and transactions with related parties
Servicing revenue								$ 100		100
Lexford Portfolio
Agreements and transactions with related parties
Income from equity affiliates								600		700
Two portfolios of multifamily properties | Consortium of Investors, including our Chief Executive Officer and our Former Manager | Maturity date of November 2018 | Bridge loans
Agreements and transactions with related parties
Principal amount						$ 14,800				$ 14,800
Base spread (as a percent)						5.25%				5.25%
LIBOR floor (as a percentage)						0.50%
Interest income recorded								$ 200		$ 200
Number of bridge loans originated | item						2
Two portfolios of multifamily properties | Certain officers, including our Chief Executive Officer and our Former Manager | Maturity date of November 2018 | Mortgage loans
Agreements and transactions with related parties
Proceeds from repayment of debt									$ 6,500
Ginkgo | Director
Agreements and transactions with related parties
Percentage of managing member							33.00%