Agreements and Transactions with Related Parties (Details 2) (USD $) Share data in Millions, unless otherwise specified	12 Months Ended		3 Months Ended	1 Months Ended	12 Months Ended	1 Months Ended	3 Months Ended	1 Months Ended				3 Months Ended	1 Months Ended					3 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2012	Jan. 31, 2014	Mar. 31, 2014	Oct. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Aug. 31, 2014 item	Sep. 30, 2014 item	Jul. 31, 2014	Nov. 30, 2013	Apr. 30, 2013 item	Jan. 31, 2013	Dec. 31, 2011	Mar. 31, 2012	Sep. 30, 2013
Agreements and transactions with related parties
Due from related party	$ 36,515	$ 98,058
Due to related party	2,653,333	2,794,087
Weighted average variable interest rate (as a percent)	5.45%	5.26%
Investments in equity affiliates	4,869,066	4,680,306
Loan amount retained by related party	35,000,000
Legal, financial and accounting fees	0	1,400,000
Additional preferred equity investment outstanding	1,587,429,498	1,660,195,301
Ownership interest allowed under company charter (as a percent)	5.00%
Subsequent event
Agreements and transactions with related parties
Amount received by related party on sale of investment in joint venture to a related party			9,600,000
Percentage of outstanding membership interest of related party in another related party			22.50%
Mezzanine Loans
Agreements and transactions with related parties
Weighted average variable interest rate (as a percent)	9.78%	7.02%
Additional preferred equity investment outstanding	76,392,650	118,550,172
Preferred equity investments
Agreements and transactions with related parties
Weighted average variable interest rate (as a percent)	6.11%	7.20%
Additional preferred equity investment outstanding	133,505,658	121,523,673
Bridge Loans
Agreements and transactions with related parties
Weighted average variable interest rate (as a percent)	5.19%	5.11%
Additional preferred equity investment outstanding	1,273,439,238	1,171,783,914
Original preferred equity investment
Agreements and transactions with related parties
Equity investment, balance	49,700,000
Maturity date of March 2016 | Bridge Loans
Agreements and transactions with related parties
Loan to third party borrower, unpaid principal balance				70,100,000
Weighted average variable interest rate (as a percent)				6.38%
Interest income recorded	3,800,000
ACM
Agreements and transactions with related parties
Due from related party	100,000	100,000
Due to related party	2,700,000	2,800,000
Number of shares held by related party	5.3
Percentage of voting power held by related party	11.00%
ACM | Subsequent event
Agreements and transactions with related parties
Percentage of non-controlling interest acquired on contribution			50.00%
ACM | Bridge Loans
Agreements and transactions with related parties
Interest income recorded	300,000	300,000			2,200,000
ACM | 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 | Maturity date of June 2014
Agreements and transactions with related parties
Number of mortgage loans secured by property purchased from related party	2
Amount of mortgage loan secured by property purchased from related party	22,400,000
Interest income recorded	900,000	1,500,000			1,500,000
Payment of debt						2,300,000
ACM | 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 | 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 | Maturity date of October 2018 | Mezzanine Loans
Agreements and transactions with related parties
Amount of mezzanine loan purchased from related party								3,000,000
Interest income recorded	300,000	100,000
Fixed rate of interest (as a percent)								13.00%
ACM | Maturity date of November 2014 | Preferred equity investments
Agreements and transactions with related parties
Loan amount retained by related party					6,000,000				6,000,000
ACM | Maturity date of November 2014 | Bridge Loans
Agreements and transactions with related parties
Loan amount purchased by related party									30,000,000
Base spread (as a percent)					5.00%				5.00%
Base rate									one-month LIBOR
Libor cap (as a percent)					1.00%				1.00%
Interest income recorded	900,000	1,600,000			200,000
ACM | Maturity date of May 2015 | Bridge Loans
Agreements and transactions with related parties
Amount of bridge loan purchased from related party										5,100,000
Base spread (as a percent)										5.50%
Base rate										one-month LIBOR
Interest income recorded	300,000	300,000			100,000
ACM | Maturity date of May 2015 | Bridge Loans | Minimum
Agreements and transactions with related parties
Libor cap (as a percent)										0.24%
Mr. Ivan Kaufman | Maturity date of August 2015 | Bridge Loans
Agreements and transactions with related parties
Investments in equity affiliates												100,000
Ownership interest (as a percent)												5.00%
Number of properties owned												2
Percentage of ownership interest of related party in the entity												95.00%
Number of bridge loans originated											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 August 2015 | Bridge Loans | Maximum
Agreements and transactions with related parties
Interest income recorded	100,000
Mr. Ivan Kaufman | Maturity date of January 2016 | 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 from related party													30,400,000
Base spread (as a percent)													7.90%
Base rate													LIBOR
Interest income recorded	1,200,000
Mr. Ivan Kaufman | ACM | 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
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															6
Base spread (as a percent)														5.50%	7.25%
Base rate														one-month LIBOR	one-month LIBOR
Interest income recorded	100,000	3,100,000
Kaufman Entities | ACM
Agreements and transactions with related parties
Percentage of outstanding membership interest of related party in another related party	92.00%
Kaufman Entities | ACM | Maturity date of March 2015 | Bridge Loans | Mr. Fred Weber
Agreements and transactions with related parties
Amount of bridge loan purchased from related party															6,400,000
Interest income recorded	200,000	200,000
Number of options to extend loan															3
Extended period of loan															1 year
Contribution by related party for acquiring non-controlling interest in a third party entity															1,100,000
Kaufman Entities | ACM | 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
Kaufman Entities | ACM | 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
Director | Maturity date of January 2015 | Bridge Loans
Agreements and transactions with related parties
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
Interest income recorded	200,000	500,000
Personal guaranty																400,000
Lexford Portfolio
Agreements and transactions with related parties
Aggregate outstanding balance of related party debt	716,100,000
Lexford Portfolio | Preferred equity investments
Agreements and transactions with related parties
Noncontrolling interest		44,000
Loan restructured																	67,600,000
Additional preferred equity investment outstanding																			22,500,000
Interest held in the additional investment made																	10,500,000
Additional investment made by the company along with a consortium of independent outside investors																	25,000,000
Equity investment made		100,000
Equity investment, balance																	0
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
Interest income recorded		1,100,000			1,300,000
Fixed rate of interest (as a percent)	2.36%
Original variable rate basis																	LIBOR
Original basis spread (as a percent)	5.00%
Modified variable rate basis	LIBOR
Modified basis spread (as a percent)	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%
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%