Agreements and Transactions with Related Parties, Other (Details) shares in Millions		1 Months Ended															3 Months Ended									6 Months Ended	12 Months Ended
	Feb. 25, 2016 USD ($)	Jan. 31, 2016 USD ($) property	Nov. 30, 2015 USD ($) item	Oct. 31, 2015 USD ($) loan	Apr. 30, 2015 USD ($)	Feb. 28, 2015 USD ($)	Aug. 31, 2014 USD ($) item	Jul. 31, 2014 USD ($)	Apr. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Jan. 31, 2014 USD ($)	Nov. 30, 2013 USD ($)	Oct. 31, 2013 USD ($)	Apr. 30, 2013 USD ($) item	Jan. 31, 2013 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2015 USD ($) shares	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($) item	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Mar. 31, 2012 USD ($)	Dec. 31, 2015 USD ($) shares	Dec. 31, 2015 USD ($) item shares	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Sep. 30, 2013 USD ($)
Agreements and transactions with related parties
Due from related party																	$ 8,082,265				$ 36,515					$ 8,082,265	$ 8,082,265	$ 36,515
Due to related party																	3,428,333				2,653,333					3,428,333	3,428,333	2,653,333
Principal loan amount																	34,300,000									34,300,000	34,300,000
Income from equity affiliates																	1,317,339	$ 6,353,239	$ 1,534,025	$ 3,095,913	219,287	$ (51,170)	$ 40,493	$ 40,048			$ 12,300,516	$ 248,658	$ (204,475)
Weighted average variable interest rate (as a percent)																											5.63%	5.45%
Investments in equity affiliates																	30,870,235				4,869,066					30,870,235	$ 30,870,235	$ 4,869,066
Unpaid Principal Balance																	1,545,126,045				1,587,429,498					1,545,126,045	1,545,126,045	1,587,429,498
Advisory fees																											3,100,000	$ 0	1,400,000
CDO II debt
Agreements and transactions with related parties
Variable interest rate, description																												three-month LIBOR
First mortgage | Multifamily
Agreements and transactions with related parties
Interest income recorded																											200,000	$ 200,000
Residential Mortgage Banking Company
Agreements and transactions with related parties
Indirect ownership percentage																				22.50%
Income from equity affiliates																											$ 6,600,000
Mezzanine Loans
Agreements and transactions with related parties
Weighted average variable interest rate (as a percent)																											8.19%	9.78%
Unpaid Principal Balance																	40,390,905				76,392,650					40,390,905	$ 40,390,905	$ 76,392,650
Preferred equity investments
Agreements and transactions with related parties
Weighted average variable interest rate (as a percent)																											7.52%	6.11%
Unpaid Principal Balance																	89,346,123				133,505,658					89,346,123	$ 89,346,123	$ 133,505,658
Bridge Loans
Agreements and transactions with related parties
Weighted average variable interest rate (as a percent)																											5.48%	5.19%
Unpaid Principal Balance																	1,353,132,435				1,273,439,238					1,353,132,435	$ 1,353,132,435	$ 1,273,439,238
Maturity date of March 2016 | Bridge Loans
Agreements and transactions with related parties
Interest income recorded																											3,800,000
Loan to third party borrower, unpaid principal balance										$ 70,100,000
Weighted average variable interest rate (as a percent)										6.38%
Maturity date of April 2016 | Preferred equity investments | Multifamily
Agreements and transactions with related parties
Fixed rate of interest (as a percent)		10.00%
Maturity date of October 2017 | Bridge Loans
Agreements and transactions with related parties
Interest income recorded																											200,000
Amount of bridge loan purchased from related party				$ 16,700,000
Number of mortgage loans secured by property purchased from related party | loan				2
Base spread (as a percent)				5.00%
Base rate				LIBOR
LIBOR floor (as a percentage)				0.25%
Maturity date of November 2018 | Bridge Loans
Agreements and transactions with related parties
Extension period			1 year
Maturity date of January 2019 | Bridge loan, two multifamily properties | Multifamily
Agreements and transactions with related parties
Variable interest rate, description		one-month LIBOR
Base spread (as a percent)		4.50%
LIBOR floor (as a percentage)		0.25%
Maturity date of January 2019 | Bridge loan, one multifamily property
Agreements and transactions with related parties
Principal loan amount		$ 19,000,000
Variable interest rate, description		one-month LIBOR
Base spread (as a percent)		4.50%
LIBOR floor (as a percentage)		0.25%
Certain Officers | Bridge loan, one multifamily property | Multifamily
Agreements and transactions with related parties
Percentage of ownership interest of related party in the entity		7.50%
Certain Officers | Maturity date of April 2016 | Preferred equity investments | Multifamily
Agreements and transactions with related parties
Equity investment		$ 5,200,000
Certain Officers | Maturity date of November 2018 | Bridge Loans
Agreements and transactions with related parties
Interest income recorded																											100,000
Amount of bridge loan purchased from related party			$ 7,100,000
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%
Number of one-year extension options | item			2
Certain Officers | Maturity date of January 2019 | Bridge loan, two multifamily properties | Multifamily
Agreements and transactions with related parties
Principal loan amount		$ 12,700,000
Number of properties owned | property		2
Percentage of ownership interest of related party in the entity		50.00%
ACM / Our "Manager" | First mortgage | Multifamily
Agreements and transactions with related parties
Principal loan amount									$ 28,500,000
Fixed rate of interest (as a percent)									14.00%
ACM / Our "Manager" | Residential Mortgage Banking Company
Agreements and transactions with related parties
Noncontrolling interest in equity method investment acquired (as a percent)																				50.00%
Acquisition purchase price																				$ 9,600,000							9,700,000
ACM / Our "Manager" | Agency Platform | Subsequent Event
Agreements and transactions with related parties
Acquisition purchase price	$ 250,000,000
ACM / Our "Manager" | Preferred equity investments | Multifamily
Agreements and transactions with related parties
Equity investment									$ 1,400,000
ACM / Our "Manager" | Maturity date of April 2025 | Mezzanine Loans | First mortgage
Agreements and transactions with related parties
Fixed rate of interest (as a percent)					12.50%
Interest income recorded																											300,000
Amount of mezzanine loan purchased from related party					$ 3,000,000
Unpaid Principal Balance					$ 47,000,000
Mr. Ivan Kaufman. | Maturity date of January 2017 | Bridge Loans
Agreements and transactions with related parties
Interest income recorded																											2,800,000	1,200,000
Percentage of ownership interest of related party in the entity								24.00%
Amount of mortgage loan secured by property purchased from related party								$ 30,400,000
Base spread (as a percent)								7.90%
Base rate								LIBOR
LIBOR floor (as a percentage)								0.50%
Mr. Ivan Kaufman. | Maturity date of March 2016 | Bridge Loans
Agreements and transactions with related parties
Interest income recorded																											300,000	100,000
Number of bridge loans originated | item							2
Amount of mortgage loan secured by property purchased from related party							$ 5,000,000
Base spread (as a percent)							5.50%
Base rate							one-month LIBOR
Mr. Ivan Kaufman. | Maturity date of March 2016 | Bridge Loans | Multifamily
Agreements and transactions with related parties
Investments in equity affiliates																						$ 100,000
Ownership interest (as a percent)																						5.00%
Number of properties owned | item																						2
Percentage of ownership interest of related party in the entity																						95.00%
Mr. Ivan Kaufman. | Maturity date of March 2015 | Bridge Loans
Agreements and transactions with related parties
Personal guaranty														$ 1,000,000
Kaufman Entities | Maturity date of April 2015 | Bridge Loans | Mr. Fred Weber
Agreements and transactions with related parties
Interest income recorded																											100,000	3,100,000
Amount of bridge loan purchased from related party												$ 2,000,000		$ 53,000,000
Percentage of ownership interest of related party in the entity														19.00%
Number of bridge loans originated | item														6
Base spread (as a percent)												5.50%		7.25%
Base rate												one-month LIBOR		one-month LIBOR
Director | Maturity Date January 2015 | Bridge Loans
Agreements and transactions with related parties
Interest income recorded																											200,000	500,000
Amount of bridge loan purchased from related party															$ 7,500,000
Percentage of ownership interest of related party in the entity															6.60%
Base spread (as a percent)															6.00%
Base rate															one-month LIBOR
Personal guaranty															$ 400,000
Lexford Portfolio | Preferred equity investments
Agreements and transactions with related parties
Distribution from equity investment																										4,500,000
Income from equity affiliates																										5,500,000
Expenses related to distribution from equity investment																										1,000,000
Noncontrolling interest																												44,000
Unpaid Principal Balance																														$ 22,500,000
Loan restructured																$ 67,600,000
Interest held in the additional investment made																10,500,000
Equity investment made																												100,000
Equity investment																0
Aggregate outstanding balance of related party debt																	$ 849,100,000									$ 849,100,000	$ 849,100,000
Lexford Portfolio | Preferred equity investments | Mr. Fred Weber
Agreements and transactions with related parties
Interest held in the additional investment made																$ 500,000
Lexford Portfolio | Original preferred equity investment
Agreements and transactions with related parties
Fixed rate of interest (as a percent)																											2.36%
Original variable rate basis																LIBOR
Original basis spread (as a percent)																	5.00%									5.00%	5.00%
Modified variable rate basis																											LIBOR
Modified basis spread (as a percent)																	1.65%									1.65%	1.65%
Lexford Portfolio | New preferred equity investment
Agreements and transactions with related parties
Fixed rate of interest (as a percent)																											12.00%
Affiliate of Mr. Ivan Kaufman
Agreements and transactions with related parties
Ownership interest (as a percent)																	53.90%									53.90%	53.90%
Contract period with the new entity																											7 years 6 months
Fees as a percentage of gross revenues of the underlying properties																											4.75%
Affiliate of Mr. Ivan Kaufman | Mr. Fred Weber
Agreements and transactions with related parties
Noncontrolling interest																									$ 250,000
Ownership interest (as a percent)																									23.50%
Mr. Ivan Kaufman and his affiliates | Maturity date of February 2018 | Preferred equity investments
Agreements and transactions with related parties
Principal loan amount						$ 23,000,000															18,000,000							18,000,000
Fixed rate of interest (as a percent)						10.00%
Interest income recorded																											$ 2,300,000
Amount of ownership interest of related party in the entity						$ 2,000,000
Mr. Ivan Kaufman and his affiliates | Maturity date of January 2017 | Mezzanine Loans
Agreements and transactions with related parties
Fixed rate of interest (as a percent)		12.00%
Amount of mortgage loan secured by property purchased from related party		$ 4,600,000
Certain officers and our Manager | Maturity date of April 2018 | Bridge Loans
Agreements and transactions with related parties
Interest income recorded																											200,000
Percentage of ownership interest of related party in the entity					90.00%
Amount of mortgage loan secured by property purchased from related party					$ 6,300,000
Base spread (as a percent)					4.50%
Interest rate, floor					0.25%
Lexford Portfolio
Agreements and transactions with related parties
Income from equity affiliates																										$ 5,500,000
Expenses related to distribution from equity investment																										1,000,000
Investments in equity affiliates																	$ 100				100					100	100	100
Noncontrolling interest																											44,000
Equity investment made																											100,000
Lexford Portfolio | Preferred equity investments
Agreements and transactions with related parties
Additional investment made by the company along with a consortium of independent outside investors																$ 25,000,000
ACM / Our "Manager"
Agreements and transactions with related parties
Due to related party																	$ 3,400,000				$ 2,700,000					$ 3,400,000	$ 3,400,000	2,700,000
Number of shares held by related party | shares																	5.3									5.3	5.3
Percentage of voting power held by related party																	10.50%									10.50%	10.50%
Ownership interest allowed under company charter (as a percent)																	5.00%									5.00%	5.00%
ACM / Our "Manager" | Bridge Loans
Agreements and transactions with related parties
Interest income recorded																											$ 300,000	1,500,000	2,200,000
ACM / Our "Manager" | Maturity date of March 2016 | Bridge Loans
Agreements and transactions with related parties
Amount of bridge loan purchased from related party										$ 15,000,000
Weighted average variable interest rate (as a percent)										7.20%
ACM / Our "Manager" | Maturity date of June 2014
Agreements and transactions with related parties
Interest income recorded																											$ 1,100,000	900,000	$ 1,500,000
Number of mortgage loans secured by property purchased from related party | item																											2
Amount of mortgage loan secured by property purchased from related party																											$ 22,400,000
Payment of debt											$ 2,300,000
ACM / Our "Manager" | Maturity date of March 2015 | First mortgage
Agreements and transactions with related parties
Amount of mortgage loan secured by property purchased from related party																								$ 14,600,000
Base spread (as a percent)										4.80%														4.80%
Base rate																								LIBOR
ACM / Our "Manager" | Maturity date of April 2015 | Second mortgage
Agreements and transactions with related parties
Amount of mortgage loan secured by property purchased from related party																								$ 5,100,000
Base spread (as a percent)										4.80%														4.80%
Base rate																								LIBOR
ACM / Our "Manager" | Maturity date of October 2018 | Mezzanine Loans
Agreements and transactions with related parties
Fixed rate of interest (as a percent)													13.00%
Interest income recorded																											300,000	100,000
Amount of mezzanine loan purchased from related party													$ 3,000,000
ACM / Our "Manager" | Kaufman Entities | Maturity date of March 2015 | Bridge Loans | Mr. Fred Weber
Agreements and transactions with related parties
Interest income recorded																											$ 200,000	$ 200,000
Amount of bridge loan purchased from related party														$ 6,400,000
Number of options to extend loan | item														3
Extended period of loan														1 year
Contribution by related party for acquiring non-controlling interest in a third party entity														$ 1,100,000
ACM / Our "Manager" | Kaufman Entities | Maturity date of March 2015 | Bridge Loans | Mr. Fred Weber | First year
Agreements and transactions with related parties
Base spread (as a percent)														5.00%
Base rate														one-month LIBOR
ACM / Our "Manager" | Kaufman Entities | Maturity date of March 2015 | Bridge Loans | Mr. Fred Weber | Thereafter
Agreements and transactions with related parties
Base spread (as a percent)														6.00%
Base rate														one-month LIBOR
Mr. Ivan Kaufman and his affiliates | ACM / Our "Manager"
Agreements and transactions with related parties
Percentage of outstanding membership interest of related party in another related party																	92.00%									92.00%	92.00%