Fair Value - Reconciliations of Assets and Liabilities Measured at Fair Value on Recurring Basis Using Significant Unobservable Inputs (Detail) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Corporate Bonds and Other [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	$ 222	$ 98
Net Realized Investment Gains (Losses) and Net Change in Unrealized Investment Gains (Losses), Included in Net Income (Loss)		(1)
Net Realized Investment Gains (Losses) and Net Change in Unrealized Investment Gains (Losses), Included in OCI	8
Purchases	56
Settlements	(2)	(2)
Transfers into Level 3		5
Transfers out of Level 3	(31)
Ending balance	253	100
Unrealized Gains (Losses) Recognized in other Comprehensive Income (Loss) on Level 3 Assets and Liabilities	7
States, Municipalities and Political Subdivisions [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		1
Ending balance		1
Total Asset-Backed [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	197	335
Net Realized Investment Gains (Losses) and Net Change in Unrealized Investment Gains (Losses), Included in Net Income (Loss)		7
Net Realized Investment Gains (Losses) and Net Change in Unrealized Investment Gains (Losses), Included in OCI	3	(5)
Purchases	20	30
Sales		(72)
Settlements	(4)	(6)
Transfers into Level 3	5
Transfers out of Level 3	(37)	(10)
Ending balance	184	279
Unrealized Gains (Losses) Recognized in other Comprehensive Income (Loss) on Level 3 Assets and Liabilities	3
Total Fixed Maturities Available-for-Sale [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	419	434
Net Realized Investment Gains (Losses) and Net Change in Unrealized Investment Gains (Losses), Included in Net Income (Loss)	0	6
Net Realized Investment Gains (Losses) and Net Change in Unrealized Investment Gains (Losses), Included in OCI	11	(5)
Purchases	76	30
Sales	0	(72)
Settlements	(6)	(8)
Transfers into Level 3	5	5
Transfers out of Level 3	(68)	(10)
Ending balance	437	380
Unrealized Gains (Losses) Recognized in other Comprehensive Income (Loss) on Level 3 Assets and Liabilities	10
Fixed Maturities Trading [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	6	4
Net Realized Investment Gains (Losses) and Net Change in Unrealized Investment Gains (Losses), Included in Net Income (Loss)	(1)	3
Ending balance	5	7
Unrealized Gains (Losses) Recognized in Net Income (Loss) on Level 3 Assets and Liabilities	(1)	3
Fixed Maturity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	425	438
Net Realized Investment Gains (Losses) and Net Change in Unrealized Investment Gains (Losses), Included in Net Income (Loss)	(1)	9
Net Realized Investment Gains (Losses) and Net Change in Unrealized Investment Gains (Losses), Included in OCI	11	(5)
Purchases	76	30
Sales	0	(72)
Settlements	(6)	(8)
Transfers into Level 3	5	5
Transfers out of Level 3	(68)	(10)
Ending balance	442	387
Unrealized Gains (Losses) Recognized in Net Income (Loss) on Level 3 Assets and Liabilities	(1)	3
Unrealized Gains (Losses) Recognized in other Comprehensive Income (Loss) on Level 3 Assets and Liabilities	10
Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	19	22
Net Realized Investment Gains (Losses) and Net Change in Unrealized Investment Gains (Losses), Included in Net Income (Loss)	2	(2)
Ending balance	21	20
Unrealized Gains (Losses) Recognized in Net Income (Loss) on Level 3 Assets and Liabilities	2	$ (2)
Equity Securities Available-for-Sale [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Unrealized Gains (Losses) Recognized in Net Income (Loss) on Level 3 Assets and Liabilities	$ 0

Amount of unrealized (holding) gain (loss) which is included in the statement of income (or changes in net assets) related to those assets still held at the reporting date for which fair value is measured on a recurring basis using significant unobservable inputs (Level 3).

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount of gain (loss) recognized in the income statement for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of gain (loss) recognized in other comprehensive income (loss) for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of sale of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of settlement of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of transfer of financial instrument classified as an asset into level 3 of the fair value hierarchy.

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Amount of transfers of financial instrument classified as an asset out of level 3 of the fair value hierarchy.

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Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of gain (loss) recognized in other comprehensive income, arising from assets, liabilities and financial instruments classified in shareholders' equity measured at fair value on a recurring basis using unobservable inputs (level 3).

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