Borrowings (Details) (USD $)	6 Months Ended	12 Months Ended	1 Months Ended			6 Months Ended	12 Months Ended					1 Months Ended	6 Months Ended			6 Months Ended							6 Months Ended					6 Months Ended				6 Months Ended					6 Months Ended
	Jun. 30, 2013	Dec. 31, 2012	Nov. 30, 2012 Securitization 2012-1 investment	Jun. 30, 2013 Securitization bonds payable	Dec. 31, 2012 Securitization bonds payable	Jun. 30, 2013 Securitization 2012-1	Dec. 31, 2012 Securitization 2012-1	Jun. 30, 2013 Credit facilities	Dec. 31, 2012 Credit facilities	Jun. 30, 2013 Loan Facilities item	Jun. 30, 2013 Loan Facilities CRE debt investments	Nov. 30, 2012 Loan Facilities Securitization 2012-1	Jun. 30, 2013 Loan Facility 1 extension	Dec. 31, 2012 Loan Facility 1	Feb. 28, 2012 Loan Facility 1	Jun. 30, 2013 Loan Facility 1 Minimum	Jun. 30, 2013 Loan Facility 1 Maximum	Jun. 30, 2013 Loan Facility 2 extension	Apr. 30, 2013 Loan Facility 2	Dec. 31, 2012 Loan Facility 2	Nov. 30, 2012 Loan Facility 2	Jul. 31, 2012 Loan Facility 2	Jun. 30, 2013 Loan Facility 2 Minimum	Jun. 30, 2013 Loan Facility 2 Maximum	Jun. 30, 2013 Loan Facility 3 extension	Dec. 31, 2012 Loan Facility 3	Jul. 31, 2012 Loan Facility 3	Jun. 30, 2013 Loan Facility 3 Minimum	Jun. 30, 2013 Loan Facility 3 Maximum	Jun. 30, 2013 Loan Facility 4 extension	Mar. 31, 2013 Loan Facility 4	Jun. 30, 2013 Loan Facility 4 Minimum	Jun. 30, 2013 Loan Facility 4 Maximum	Jun. 30, 2013 CMBS Facilities	Dec. 31, 2012 CMBS Facilities	Sep. 30, 2012 CMBS Facilities agreement	Jun. 30, 2013 CMBS Facilities Minimum	Jun. 30, 2013 CMBS Facilities Maximum
Borrowings
Contractual Interest Rate (as a percent)																																		1.45%	1.45%
Weighted average interest rate (as a percent)													2.95%	2.95%				3.44%		3.44%					4.90%	4.90%				5.20%
Principal Amount	$ 350,530,000	$ 250,980,000		$ 124,659,000	$ 124,659,000	$ 124,659,000	$ 124,659,000	$ 225,871,000	$ 126,321,000				$ 32,836,000	$ 32,836,000				$ 100,000,000		$ 54,950,000					$ 35,850,000	$ 26,850,000				$ 45,500,000				$ 11,685,000	$ 11,685,000
Carrying Value	350,481,000	250,812,000		124,610,000	124,491,000	124,610,000	124,491,000	225,871,000	126,321,000	214,200,000			32,836,000	32,836,000				100,000,000		54,950,000					35,850,000	26,850,000				45,500,000				11,685,000	11,685,000
Percentage of core assets for which recourse is available													25.00%
Percentage of flex assets for which recourse is available													100.00%
Percentage of line of credit for which recourse may be available													25.00%
Number of optional extensions to initial maturity date available to the entity													2					3							3					4
Optional extension period of initial maturity date													1 year					1 year							1 year					1 year
Variable rate basis			LIBOR			LIBOR	LIBOR						one-month LIBOR					one-month LIBOR							one-month LIBOR					one-month LIBOR
Interest rate margin (as a percent)			1.63%			1.66%	1.66%									2.50%	3.00%						2.00%	4.00%				3.95%	5.95%			2.50%	3.00%
Percentage of repurchase price for purchased assets for which a recourse is available for debt yielding equal to or greater than 10% at the time of financing																		25.00%
Percentage of repurchase price for purchased assets for which a recourse is available for debt yield less than 10% at the time of financing																		100.00%
Percentage of financed amount of stabilized loans for which recourse is available																														25.00%
Financed amount of loans for which recourse is available																														25,000,000
Maturity period of debt instruments depending upon the asset type																																					2 months	3 months
Scheduled principal payment requirements on borrowings
July 1 - December 31, 2013	11,685,000							11,685,000
2016	32,836,000							32,836,000
2017	100,000,000							100,000,000
Thereafter	206,009,000			124,659,000				81,350,000
Total	350,530,000	250,980,000		124,659,000	124,659,000	124,659,000	124,659,000	225,871,000	126,321,000				32,836,000	32,836,000				100,000,000		54,950,000					35,850,000	26,850,000				45,500,000				11,685,000	11,685,000
Principal amount of investments financed by borrowings	945,910,000	543,619,000	351,400,000								360,300,000
Number of CRE debt investments contributed			9
Aggregate principal amount of CRE debt investment contributed			199,200,000
Equity interest retained in CRE debt investments contributed			70,000,000
Value of bonds issued			227,500,000
Value of bonds used to finance the assets contributed			129,500,000
Advance rates (as a percent)			65.00%
Description of variable rate	LIBOR	LIBOR
Amount of borrowings repaid												117,700,000
Number of facilities										4
Maximum borrowing capacity															100,000,000				150,000,000		100,000,000	50,000,000					40,000,000				200,000,000
Unrestricted cash or cash equivalents required to be maintained under financial covenants																$ 5,000,000	$ 15,000,000						$ 3,800,000	$ 22,500,000				$ 3,800,000	$ 7,500,000			$ 20,000,000
Number of master repurchase agreements entered into																																				2