Percentage Vintage, Property Type, and Location of Collateral Securing Company's AAA-Rated CMBS Investments Calculated on Weighted Average Basis (Detail)	9 Months Ended	12 Months Ended
	Sep. 30, 2012	Dec. 31, 2011
Mortgage Loans on Real Estate [Line Items]
Percentage Vintage	100.00%	100.00%
South Atlantic
Mortgage Loans on Real Estate [Line Items]
Percentage Vintage	22.10%	23.00%
Middle Atlantic
Mortgage Loans on Real Estate [Line Items]
Percentage Vintage	21.30%	21.90%
Pacific
Mortgage Loans on Real Estate [Line Items]
Percentage Vintage	23.90%	21.00%
Other
Mortgage Loans on Real Estate [Line Items]
Percentage Vintage	32.70%	34.10%
Office Complex - Missouri
Mortgage Loans on Real Estate [Line Items]
Percentage Vintage	40.50%	36.50%
Retail
Mortgage Loans on Real Estate [Line Items]
Percentage Vintage	23.30%	26.60%
Multifamily
Mortgage Loans on Real Estate [Line Items]
Percentage Vintage	12.90%	13.10%
Hotel
Mortgage Loans on Real Estate [Line Items]
Percentage Vintage	10.40%	10.80%
Other
Mortgage Loans on Real Estate [Line Items]
Percentage Vintage	12.90%	13.00%
2006
Mortgage Loans on Real Estate [Line Items]
Percentage Vintage	3.00%	7.00%
2007
Mortgage Loans on Real Estate [Line Items]
Percentage Vintage	97.00%	93.00%