Commercial Mortgage Residual Interests Commercial Mortgage Residual Interests (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended			12 Months Ended
	Mar. 31, 2013 securitizations		Mar. 31, 2012	Dec. 31, 2012
Mortgage Loans on Real Estate [Line Items]
Number of securitizations	7
Discount rate	25.00%			25.00%
Loss severity of loans in foreclosure	20.00%			20.00%
LIBOR	Forward 3-month curve			Forward 3-month curve
Prime	Forward curve			Forward curve
Unrealized gains	$ 869		$ 0
Maximum [Member]
Mortgage Loans on Real Estate [Line Items]
Average life equivalent CPR speeds range	25.04%	[1]		24.31%	[1]
Foreclosures frequency curve default model	4.76%			4.49%
Minimum [Member]
Mortgage Loans on Real Estate [Line Items]
Average life equivalent CPR speeds range	0.80%	[1]		0.08%	[1]
Foreclosures frequency curve default model	0.08%			0.09%

[1] Conditional prepayment rate.