Total Investments in Bank and Financial Institution Sector, Including those Classified as Perpetual Securities (Detail) (Banks/financial Institutions, USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Summary of Investment Holdings [Line Items]
Amortized Cost	$ 26,578	$ 29,540
Fair Value	25,011	27,910
Percentage of Total Investment Portfolio, Amortized Cost	27.00%	35.00%
Percentage of Total Investment Portfolio, Fair Value	25.00%	32.00%
Debt Securities
Summary of Investment Holdings [Line Items]
Amortized Cost	20,025	22,041
Fair Value	18,933	20,741
Percentage of Total Investment Portfolio, Amortized Cost	20.00%	26.00%
Percentage of Total Investment Portfolio, Fair Value	19.00%	24.00%
Perpetual Securities | Upper Tier II
Summary of Investment Holdings [Line Items]
Amortized Cost	4,285	4,957
Fair Value	4,244	4,748
Percentage of Total Investment Portfolio, Amortized Cost	5.00%	6.00%
Percentage of Total Investment Portfolio, Fair Value	4.00%	5.00%
Perpetual Securities | Tier I
Summary of Investment Holdings [Line Items]
Amortized Cost	2,268	2,542
Fair Value	$ 1,834	$ 2,421
Percentage of Total Investment Portfolio, Amortized Cost	2.00%	3.00%
Percentage of Total Investment Portfolio, Fair Value	2.00%	3.00%