Summary of Significant Inputs Used by Third-Party Pricing Services for Certain Fair Value Measurements of Fixed Maturity Securities that Classified as Level 2 (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Fair value measurements Significant unobservable inputs [Line Items]
Available-for-sale debt securities	$ 62,552	$ 60,572
Fixed maturity securities | U.S. government, agencies and government-sponsored enterprises
Fair value measurements Significant unobservable inputs [Line Items]
Available-for-sale debt securities	5,670	6,036
Fixed maturity securities | State and Political Subdivisions
Fair value measurements Significant unobservable inputs [Line Items]
Available-for-sale debt securities	2,860	2,647
Fixed maturity securities | Non-U.S. government
Fair value measurements Significant unobservable inputs [Line Items]
Available-for-sale debt securities	2,226	2,107
Fixed maturity securities | U.S. corporate
Fair value measurements Significant unobservable inputs [Line Items]
Available-for-sale debt securities	28,482	26,828
Fixed maturity securities | Non-U.S. corporate
Fair value measurements Significant unobservable inputs [Line Items]
Available-for-sale debt securities	12,623	12,295
Fixed maturity securities | Residential mortgage-backed
Fair value measurements Significant unobservable inputs [Line Items]
Available-for-sale debt securities	4,209	4,379
Fixed maturity securities | Commercial mortgage-backed
Fair value measurements Significant unobservable inputs [Line Items]
Available-for-sale debt securities	3,414	3,129
Fixed maturity securities | Other asset-backed
Fair value measurements Significant unobservable inputs [Line Items]
Available-for-sale debt securities	3,068	3,151
Level 2
Fair value measurements Significant unobservable inputs [Line Items]
Available-for-sale debt securities	58,403	56,271
Level 2 | Fixed maturity securities | U.S. government, agencies and government-sponsored enterprises
Fair value measurements Significant unobservable inputs [Line Items]
Available-for-sale debt securities	5,669	6,034
Level 2 | Fixed maturity securities | State and Political Subdivisions
Fair value measurements Significant unobservable inputs [Line Items]
Available-for-sale debt securities	2,823	2,610
Level 2 | Fixed maturity securities | Non-U.S. government
Fair value measurements Significant unobservable inputs [Line Items]
Available-for-sale debt securities	2,226	2,107
Level 2 | Fixed maturity securities | U.S. corporate
Fair value measurements Significant unobservable inputs [Line Items]
Available-for-sale debt securities	26,155	24,341
Level 2 | Fixed maturity securities | Non-U.S. corporate
Fair value measurements Significant unobservable inputs [Line Items]
Available-for-sale debt securities	11,172	10,762
Level 2 | Fixed maturity securities | Residential mortgage-backed
Fair value measurements Significant unobservable inputs [Line Items]
Available-for-sale debt securities	4,123	4,336
Level 2 | Fixed maturity securities | Commercial mortgage-backed
Fair value measurements Significant unobservable inputs [Line Items]
Available-for-sale debt securities	3,392	3,075
Level 2 | Fixed maturity securities | Other asset-backed
Fair value measurements Significant unobservable inputs [Line Items]
Available-for-sale debt securities	2,843	$ 3,006
Level 2 | Third-Party Pricing Services | Fixed maturity securities | U.S. government, agencies and government-sponsored enterprises
Fair value measurements Significant unobservable inputs [Line Items]
Available-for-sale debt securities	$ 5,669
Primary methodologies	Price quotes from trading desk, broker feeds
Significant inputs	Bid side prices, trade prices, Option Adjusted Spread (“OAS”) to swap curve, Bond Market Association OAS, Treasury Curve, Agency Bullet Curve, maturity to issuer spread
Level 2 | Third-Party Pricing Services | Fixed maturity securities | State and Political Subdivisions
Fair value measurements Significant unobservable inputs [Line Items]
Available-for-sale debt securities	$ 2,816
Primary methodologies	Multi-dimensional attribute-based modeling systems, third-party pricing vendors
Significant inputs	Trade prices, material event notices, Municipal Market Data benchmark yields, broker quotes
Level 2 | Third-Party Pricing Services | Fixed maturity securities | Non-U.S. government
Fair value measurements Significant unobservable inputs [Line Items]
Available-for-sale debt securities	$ 2,210
Primary methodologies	Matrix pricing, spread priced to benchmark curves, price quotes from market makers
Significant inputs	Benchmark yields, trade prices, broker quotes, comparative transactions, issuer spreads, bid-offer spread, market research publications, third-party pricing sources
Level 2 | Third-Party Pricing Services | Fixed maturity securities | U.S. corporate
Fair value measurements Significant unobservable inputs [Line Items]
Available-for-sale debt securities	$ 25,290
Primary methodologies	Multi-dimensional attribute-based modeling systems, broker quotes, price quotes from market makers, internal models, OAS-basedmodels
Significant inputs	Bid side prices to Treasury Curve, Issuer Curve, which includes sector, quality, duration, OAS percentage and change for spread matrix, trade prices, comparative transactions, Trade Reporting and Compliance Engine ("TRACE") reports
Level 2 | Third-Party Pricing Services | Fixed maturity securities | Non-U.S. corporate
Fair value measurements Significant unobservable inputs [Line Items]
Available-for-sale debt securities	$ 10,711
Primary methodologies	Multi-dimensional attribute-based modeling systems, OAS-based models, price quotes from market makers
Significant inputs	Benchmark yields, trade prices, broker quotes, comparative transactions, issuer spreads, bid-offer spread, market research publications, third-party pricing sources
Level 2 | Third-Party Pricing Services | Fixed maturity securities | Residential mortgage-backed
Fair value measurements Significant unobservable inputs [Line Items]
Available-for-sale debt securities	$ 4,123
Primary methodologies	OAS-based models, To Be Announced pricing models, single factor binomial models, internally priced
Significant inputs	Prepayment and default assumptions, aggregation of bonds with similar characteristics, including collateral type, vintage, tranche type, weighted-average life, weighted-average loan age, issuer program and delinquency ratio, pay up and pay down factors, TRACE reports
Level 2 | Third-Party Pricing Services | Fixed maturity securities | Commercial mortgage-backed
Fair value measurements Significant unobservable inputs [Line Items]
Available-for-sale debt securities	$ 3,392
Primary methodologies	Multi-dimensional attribute-based modeling systems, pricing matrix, spread matrix priced to swap curves, Trepp commercial mortgage-backed securities analytics model
Significant inputs	Credit risk, interest rate risk, prepayment speeds, new issue data, collateral performance, origination year, tranche type, original credit ratings, weighted-average life, cash flows, spreads derived from broker quotes, bid side prices, spreads to daily updated swaps curves, TRACE reports
Level 2 | Third-Party Pricing Services | Fixed maturity securities | Other asset-backed
Fair value measurements Significant unobservable inputs [Line Items]
Available-for-sale debt securities	$ 2,843
Primary methodologies	Multi-dimensional attribute-based modeling systems, spread matrix priced to swap curves, price quotes from market makers, internal models
Significant inputs	Spreads to daily updated swaps curves, spreads derived from trade prices and broker quotes, bid side prices, new issue data, collateral performance, analysis of prepayment speeds, cash flows, collateral loss analytics, historical issue analysis, trade data from market makers, TRACE reports