Invested Assets and Related Income - Summary of Fair Value of Tax Exempt Fixed Maturities (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Fixed maturities, Fair Value	$ 38,780	$ 37,091
Tax Exempt [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Fixed maturities, Fair Value	19,772	18,421
Tax Exempt [Member] | Tax Exempt Fixed Maturities [Member] | Special Revenue Bonds [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Fixed maturities, Fair Value	12,936	11,717
Tax Exempt [Member] | Tax Exempt Fixed Maturities [Member] | State General Obligation Bonds [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Fixed maturities, Fair Value	2,415	2,292
Tax Exempt [Member] | Tax Exempt Fixed Maturities [Member] | Municipal and Political Subdivision General Obligation Bonds [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Fixed maturities, Fair Value	2,378	2,220
Tax Exempt [Member] | Tax Exempt Fixed Maturities [Member] | Pre-Refunded Bonds [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Fixed maturities, Fair Value	$ 2,043	$ 2,192