Investments in fixed maturity securities - Amortized cost and estimated fair value of securities with fixed maturities (Detail) (Fixed Maturities [Member], USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Summary of Investment Holdings [Line Items]
Due in one year or less - amortized cost	$ 7,650
Due after one year through five years - amortized cost	11,341
Due after five years through ten years - amortized cost	2,782
Due after ten years - amortized cost	2,695
Amortized cost of fixed maturity securities	26,023	27,764
Due in one year or less - fair value	7,585
Due after one year through five years - fair value	11,994
Due after five years through ten years - fair value	3,009
Due after ten years - fair value	3,295
Fair value of fixed maturity securities	27,636	29,353
Mortgage-backed securities [Member]
Summary of Investment Holdings [Line Items]
Amortized cost of mortgage-backed securities	1,555
Amortized cost of fixed maturity securities	1,555	1,830
Fair value of mortgage-backed securities	1,753
Fair value of fixed maturity securities	$ 1,753	$ 2,040

Investments in fixed maturity securities - Amortized cost and estimated fair value of securities with fixed maturities (Detail) (Fixed Maturities [Member], USD $)
In Millions, unless otherwise specified

Dec. 31, 2014

Dec. 31, 2013

Summary of Investment Holdings [Line Items]

Due in one year or less - amortized cost

$ 7,650us-gaap_AvailableForSaleSecuritiesDebtMaturitiesNextRollingTwelveMonthsAmortizedCostBasis

Due after one year through five years - amortized cost

11,341us-gaap_AvailableForSaleSecuritiesDebtMaturitiesRollingYearTwoThroughFiveAmortizedCostBasis

Due after five years through ten years - amortized cost

2,782us-gaap_AvailableForSaleSecuritiesDebtMaturitiesRollingYearSixThroughTenAmortizedCostBasis

Due after ten years - amortized cost

2,695us-gaap_AvailableForSaleSecuritiesDebtMaturitiesRollingAfterYearTenAmortizedCostBasis

Amortized cost of fixed maturity securities

26,023us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis

27,764us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis

Due in one year or less - fair value

7,585us-gaap_AvailableForSaleSecuritiesDebtMaturitiesNextRollingTwelveMonthsFairValue

Due after one year through five years - fair value

11,994us-gaap_AvailableForSaleSecuritiesDebtMaturitiesRollingYearTwoThroughFiveFairValue

Due after five years through ten years - fair value

3,009us-gaap_AvailableForSaleSecuritiesDebtMaturitiesRollingYearSixThroughTenFairValue

Due after ten years - fair value

3,295us-gaap_AvailableForSaleSecuritiesDebtMaturitiesRollingAfterYearTenFairValue

Fair value of fixed maturity securities

27,636us-gaap_AvailableForSaleSecuritiesDebtSecurities

29,353us-gaap_AvailableForSaleSecuritiesDebtSecurities

Mortgage-backed securities [Member]

Summary of Investment Holdings [Line Items]

Amortized cost of mortgage-backed securities

1,555us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateAmortizedCost
/ us-gaap_InvestmentSecondaryCategorizationAxis
= us-gaap_MortgageBackedSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember

Amortized cost of fixed maturity securities

1,555us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_InvestmentSecondaryCategorizationAxis
= us-gaap_MortgageBackedSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember

1,830us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_InvestmentSecondaryCategorizationAxis
= us-gaap_MortgageBackedSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember

Fair value of mortgage-backed securities

1,753us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateFairValue
/ us-gaap_InvestmentSecondaryCategorizationAxis
= us-gaap_MortgageBackedSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember

Fair value of fixed maturity securities

$ 1,753us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_InvestmentSecondaryCategorizationAxis
= us-gaap_MortgageBackedSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember

$ 2,040us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_InvestmentSecondaryCategorizationAxis
= us-gaap_MortgageBackedSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember

- Definition

This item represents the cost of debt securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.2)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (aa)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563

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- Definition

Amount after adjustments of available-for-sale debt securities at cost, maturing in the next rolling twelve months following the latest balance sheet. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings less cumulative-effect adjustments and fair value hedge accounting adjustments. For interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

No definition available.

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- Definition

Fair value of available-for-sale debt securities maturing in the next rolling twelve months following the latest balance sheet. For interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

No definition available.

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- Definition

Amount after adjustments of available-for-sale debt securities at cost, maturing after the tenth rolling twelve months following the latest balance sheet. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings less cumulative-effect adjustments, and fair value hedge accounting adjustments. For interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

No definition available.

+ Details

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- Definition

Fair value of available-for-sale debt securities maturing after the tenth rolling twelve months following the latest balance sheet. For interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

No definition available.

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- Definition

Amount after adjustments of available-for-sale debt securities at cost, maturing in the sixth through tenth rolling twelve months following the latest balance sheet. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings less cumulative-effect adjustments, and fair value hedge accounting adjustments. For interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value of available-for-sale debt securities maturing in the sixth through tenth rolling twelve months following the latest balance sheet. For interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

No definition available.

+ Details

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- Definition

Amount after adjustments of available-for-sale debt securities at cost, maturing in the second through fifth rolling twelve months following the latest balance sheet. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings less cumulative-effect adjustments, and fair value hedge accounting adjustments. For interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

No definition available.

+ Details

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- Definition

Fair value of available-for-sale debt securities maturing in the second through fifth rolling twelve months following the latest balance sheet. For interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

No definition available.

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- Definition

Amount of available-for-sale debt securities at cost, net of adjustments, that do not have a single maturity date and which the entity has decided to disclose separately rather than allocating the cost over several maturity groupings. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings (less any cumulative-effect adjustments, as defined) and fair value hedge accounting adjustments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.2)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 3

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27198-111563

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- Definition

Amount of available-for-sale debt securities at fair value that do not have a single maturity date and which the entity has decided to disclose separately rather than allocating the fair value over several maturity groupings.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 3

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27198-111563

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (aa)

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- Definition

Amount of debt securities categorized neither as held-to-maturity nor trading.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (aa)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 320

-Section 50

-Paragraph 2

-Subparagraph (b)-(g)

-URI http://asc.fasb.org/extlink&oid=6957658&loc=d3e62557-112803

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