Percentage Vintage, Property Type, and Location of Collateral Securing Company's AAA-Rated CMBS Investments Calculated on Weighted Average Basis (Detail)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Mortgage Loans on Real Estate [Line Items]
Percentage Vintage	100.00%	100.00%
South Atlantic [Member]
Mortgage Loans on Real Estate [Line Items]
Percentage Vintage	21.80%	23.00%
Middle Atlantic [Member]
Mortgage Loans on Real Estate [Line Items]
Percentage Vintage	21.40%	21.90%
Pacific [Member]
Mortgage Loans on Real Estate [Line Items]
Percentage Vintage	23.80%	21.00%
Other [Member]
Mortgage Loans on Real Estate [Line Items]
Percentage Vintage	33.00%	34.10%
Office [Member]
Mortgage Loans on Real Estate [Line Items]
Percentage Vintage	40.50%	36.50%
Retail [Member]
Mortgage Loans on Real Estate [Line Items]
Percentage Vintage	23.20%	26.60%
Multifamily [Member]
Mortgage Loans on Real Estate [Line Items]
Percentage Vintage	12.90%	13.10%
Hotel [Member]
Mortgage Loans on Real Estate [Line Items]
Percentage Vintage	10.50%	10.80%
Other [Member]
Mortgage Loans on Real Estate [Line Items]
Percentage Vintage	12.90%	13.00%
2006 [Member]
Mortgage Loans on Real Estate [Line Items]
Percentage Vintage	1.00%	7.00%
2007 [Member]
Mortgage Loans on Real Estate [Line Items]
Percentage Vintage	99.00%	93.00%