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Debt Securities - Percentage Vintage, Property Type, and Location of Collateral Securing Company's AAA-Rated CMBS Investments Calculated on Weighted Average Basis (Detail)	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Mortgage Loans on Real Estate [Line Items]
Percentage Vintage	100.00%	100.00%
Middle Atlantic [Member]
Mortgage Loans on Real Estate [Line Items]
Percentage Vintage	22.70%	21.40%
South Atlantic [Member]
Mortgage Loans on Real Estate [Line Items]
Percentage Vintage	22.50%	21.80%
Pacific [Member]
Mortgage Loans on Real Estate [Line Items]
Percentage Vintage	22.10%	23.80%
Other [Member]
Mortgage Loans on Real Estate [Line Items]
Percentage Vintage	32.70%	33.00%
Office [Member]
Mortgage Loans on Real Estate [Line Items]
Percentage Vintage	41.00%	40.50%
Retail [Member]
Mortgage Loans on Real Estate [Line Items]
Percentage Vintage	22.90%	23.20%
Multifamily [Member]
Mortgage Loans on Real Estate [Line Items]
Percentage Vintage	12.20%	12.90%
Hotel [Member]
Mortgage Loans on Real Estate [Line Items]
Percentage Vintage	10.70%	10.50%
Other [Member]
Mortgage Loans on Real Estate [Line Items]
Percentage Vintage	13.20%	12.90%
2006 [Member]
Mortgage Loans on Real Estate [Line Items]
Percentage Vintage		1.00%
2007 [Member]
Mortgage Loans on Real Estate [Line Items]
Percentage Vintage	100.00%	99.00%