Commercial Mortgage Residual Interests Commercial Mortgage Residual Interests (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended			12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012 securitizations		Sep. 30, 2011	Dec. 31, 2011
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			$ 213		$ 599
Other than temporary valuation impairment	$ 0	$ 0	$ 2,718		$ 396
Maximum [Member]
Mortgage Loans on Real Estate [Line Items]
Average life equivalent CPR speeds range			20.42%	[1]		18.57%	[1]
Foreclosures frequency curve default model			3.90%			4.70%
Minimum [Member]
Mortgage Loans on Real Estate [Line Items]
Average life equivalent CPR speeds range			0.80%	[1]		2.18%	[1]
Foreclosures frequency curve default model			0.10%			0.20%

[1] Conditional prepayment rate.