Borrowings (Details) (USD $)	3 Months Ended	12 Months Ended		1 Months Ended	3 Months Ended		1 Months Ended							3 Months Ended			3 Months Ended		3 Months Ended					3 Months Ended				3 Months Ended			3 Months Ended					3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2013 Forecast	Nov. 30, 2012 Securitization investment	Mar. 31, 2013 Securitization Minimum	Mar. 31, 2013 Securitization Maximum	Nov. 30, 2012 Securitization bonds payable	Mar. 31, 2013 Securitization bonds payable	Dec. 31, 2012 Securitization bonds payable	Mar. 31, 2013 Securitization 2012-1	Dec. 31, 2012 Securitization 2012-1	Mar. 31, 2013 Credit facilities	Dec. 31, 2012 Credit facilities	Sep. 30, 2012 Loan Facilities	Mar. 31, 2013 Loan Facilities item	Mar. 31, 2013 Loan Facilities CRE debt investments	Mar. 31, 2013 Loan Facility 1 extension	Dec. 31, 2012 Loan Facility 1	Mar. 31, 2013 Loan Facility 1 Minimum	Mar. 31, 2013 Loan Facility 1 Maximum	Mar. 31, 2013 Loan Facility 2 extension	Dec. 31, 2012 Loan Facility 2	Jul. 31, 2012 Loan Facility 2	Mar. 31, 2013 Loan Facility 2 Minimum	Mar. 31, 2013 Loan Facility 2 Maximum	Mar. 31, 2013 Loan Facility 2 Debt yield equal to or greater than 10% at the time of financing	Mar. 31, 2013 Loan Facility 2 Debt yield less than 10% at the time of financing	Mar. 31, 2013 Loan Facility 3 extension	Dec. 31, 2012 Loan Facility 3	Jul. 31, 2012 Loan Facility 3	Mar. 31, 2013 Loan Facility 3 Minimum	Mar. 31, 2013 Loan Facility 3 Maximum	Mar. 31, 2013 Loan Facility 4 extension	Mar. 31, 2013 Loan Facility 4 Minimum	Mar. 31, 2013 Loan Facility 4 Maximum	Mar. 31, 2013 CMBS Facilities	Dec. 31, 2012 CMBS Facilities	Sep. 30, 2012 CMBS Facilities agreement
Borrowings
Contractual Interest Rate (as a percent)										1.87%	1.87%																									1.57%	1.57%
Weighted average interest rate (as a percent)																	2.95%	2.95%			3.45%	3.45%						4.91%	4.91%
Principal Amount	$ 305,030,000	$ 250,980,000						$ 124,659,000	$ 124,659,000	$ 124,659,000	$ 124,659,000	$ 180,371,000	$ 126,321,000				$ 32,836,000	$ 32,836,000			$ 100,000,000	$ 54,950,000						$ 35,850,000	$ 26,850,000							$ 11,685,000	$ 11,685,000
Carrying Value	304,930,000	250,812,000						124,559,000	124,491,000	124,559,000	124,491,000	180,371,000	126,321,000		168,700,000		32,836,000	32,836,000			100,000,000	54,950,000						35,850,000	26,850,000							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																	one-month LIBOR				one-month LIBOR							one-month LIBOR					one-month LIBOR
Interest rate margin (as a percent)																			2.50%	3.00%				2.00%	4.00%						3.95%	5.95%		2.50%	3.00%
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
April 1 to December 31, 2013	11,685,000											11,685,000
2016	32,836,000											32,836,000
2017	100,000,000											100,000,000
Thereafter	160,509,000							124,659,000				35,850,000
Total	305,030,000	250,980,000						124,659,000	124,659,000	124,659,000	124,659,000	180,371,000	126,321,000				32,836,000	32,836,000			100,000,000	54,950,000						35,850,000	26,850,000							11,685,000	11,685,000
Percentage of repurchase price for purchased assets for which a recourse is available																										25.00%	10.00%
Principal amount of investments financed by borrowings	649,613,000	543,619,000		351,400,000												304,100,000
Real estate debt investments, net	624,377,000	514,058,000
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		LIBOR
Spread over LIBOR (as a percent)				1.63%
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
Yield (as a percent)			15.00%
Amount of borrowings outstanding														117,700,000																						16,600,000
Number of facilities															3
Maximum borrowing capacity																	100,000,000				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	15,000,000						3,800,000	7,500,000		20,000,000
Borrowings on loan facility outstanding																																	$ 0
Number of master repurchase agreements entered into																																						2