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	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Fair value measurements Significant unobservable inputs [Line Items]
Available-for-sale debt securities	$ 61,360	$ 59,661
Fixed maturity securities | U.S. government, agencies and government-sponsored enterprises
Fair value measurements Significant unobservable inputs [Line Items]
Available-for-sale debt securities	4,731	4,631
Fixed maturity securities | State and Political Subdivisions
Fair value measurements Significant unobservable inputs [Line Items]
Available-for-sale debt securities	2,546	2,552
Fixed maturity securities | Non-U.S. government
Fair value measurements Significant unobservable inputs [Line Items]
Available-for-sale debt securities	2,518	2,393
Fixed maturity securities | U.S. corporate
Fair value measurements Significant unobservable inputs [Line Items]
Available-for-sale debt securities	29,942	28,762
Fixed maturity securities | Non-U.S. corporate
Fair value measurements Significant unobservable inputs [Line Items]
Available-for-sale debt securities	12,285	11,837
Fixed maturity securities | Residential mortgage-backed
Fair value measurements Significant unobservable inputs [Line Items]
Available-for-sale debt securities	2,950	3,044
Fixed maturity securities | Commercial mortgage-backed
Fair value measurements Significant unobservable inputs [Line Items]
Available-for-sale debt securities	2,962	3,016
Fixed maturity securities | Other asset-backed
Fair value measurements Significant unobservable inputs [Line Items]
Available-for-sale debt securities	3,426	3,426
Level 2
Fair value measurements Significant unobservable inputs [Line Items]
Available-for-sale debt securities	57,236	55,785
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	4,731	4,631
Level 2 | Fixed maturity securities | State and Political Subdivisions
Fair value measurements Significant unobservable inputs [Line Items]
Available-for-sale debt securities	2,494	2,501
Level 2 | Fixed maturity securities | Non-U.S. government
Fair value measurements Significant unobservable inputs [Line Items]
Available-for-sale debt securities	2,518	2,393
Level 2 | Fixed maturity securities | U.S. corporate
Fair value measurements Significant unobservable inputs [Line Items]
Available-for-sale debt securities	27,780	26,764
Level 2 | Fixed maturity securities | Non-U.S. corporate
Fair value measurements Significant unobservable inputs [Line Items]
Available-for-sale debt securities	10,710	10,305
Level 2 | Fixed maturity securities | Residential mortgage-backed
Fair value measurements Significant unobservable inputs [Line Items]
Available-for-sale debt securities	2,915	3,009
Level 2 | Fixed maturity securities | Commercial mortgage-backed
Fair value measurements Significant unobservable inputs [Line Items]
Available-for-sale debt securities	2,864	2,921
Level 2 | Fixed maturity securities | Other asset-backed
Fair value measurements Significant unobservable inputs [Line Items]
Available-for-sale debt securities	3,224	$ 3,261
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	$ 4,731
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	$ 2,494
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	$ 2,502
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	$ 26,748
Primary methodologies	Multi-dimensional attribute-based modeling systems, broker quotes, price quotes from market makers, internal models, OAS-based models
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	$ 10,123
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	$ 2,915
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	$ 2,864
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	$ 3,224
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