Real Estate Debt Investments (Details) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2013 investment	Dec. 31, 2012 investment
CRE debt investments
Number	22	20
Principal Amount	$ 649,613,000	$ 543,619,000
Carrying Value	624,377,000	514,058,000
Description of variable rate	LIBOR	LIBOR
Floating Rate as % of Principal Amount	82.30%	81.00%
Securitization Financing
CRE debt investments
Carrying value served as collateral	194,500,000	194,500,000
Term Loan Facilities
CRE debt investments
Carrying value served as collateral	304,800,000	197,600,000
Weighted Average
CRE debt investments
Fixed Rate (as a percent)	12.37%	12.47%
Spread over LIBOR (as a percent)	6.36%	6.18%
Unleveraged Current Yield (as a percent)	8.42%	8.50%
LIBOR floor (as a percent)	1.38%	1.61%
First mortgage / mezzanine loans
CRE debt investments
Future funding commitments	25,900,000	29,800,000
First mortgage loans
CRE debt investments
Number	20	18
Principal Amount	593,113,000	486,992,000
Carrying Value	583,306,000	476,717,000
Description of variable rate	LIBOR	LIBOR
Floating Rate as % of Principal Amount	89.40%	89.50%
First mortgage loans | Weighted Average
CRE debt investments
Fixed Rate (as a percent)	12.98%	13.25%
Spread over LIBOR (as a percent)	6.33%	6.14%
Unleveraged Current Yield (as a percent)	8.16%	8.22%
LIBOR floor (as a percent)	1.38%	1.61%
Mezzanine loans
CRE debt investments
Number	2	2
Principal Amount	56,500,000	56,627,000
Carrying Value	$ 41,071,000	$ 37,341,000
Description of variable rate	LIBOR	LIBOR
Floating Rate as % of Principal Amount	8.00%	8.20%
Mezzanine loans | Weighted Average
CRE debt investments
Fixed Rate (as a percent)	11.50%	11.50%
Spread over LIBOR (as a percent)	10.00%	10.00%
Unleveraged Current Yield (as a percent)	12.05%	12.12%
LIBOR floor (as a percent)	1.38%	1.61%