Invested Assets and Related Income - Summary of Fair Value of Tax Exempt Fixed Maturities (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Schedule of Available-For-Sale Securities [Line Items]
Fair value of the tax exempt fixed maturities	$ 19,913	$ 20,211
Tax exempt fixed maturities [Member] | Special revenue bonds [Member]
Schedule of Available-For-Sale Securities [Line Items]
Fair value of the tax exempt fixed maturities	12,233	12,405
Tax exempt fixed maturities [Member] | Municipal and political subdivision general obligation bonds [Member]
Schedule of Available-For-Sale Securities [Line Items]
Fair value of the tax exempt fixed maturities	2,505	2,614
Tax exempt fixed maturities [Member] | State general obligation bonds [Member]
Schedule of Available-For-Sale Securities [Line Items]
Fair value of the tax exempt fixed maturities	2,246	2,548
Tax exempt fixed maturities [Member] | Pre-refunded bonds [Member]
Schedule of Available-For-Sale Securities [Line Items]
Fair value of the tax exempt fixed maturities	$ 2,929	$ 2,644