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	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Corporate Bonds and Other [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	$ 253	$ 100	$ 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	12	(1)	20	(1)
Purchases	76	2	132	2
Sales		(5)		(5)
Settlements	(2)	(2)	(4)	(4)
Transfers into Level 3				5
Transfers out of Level 3	(1)		(32)
Ending balance	338	94	338	94
Unrealized Gains (Losses) Recognized in other Comprehensive Income (Loss) on Level 3 Assets and Liabilities	10		17
States, Municipalities and Political Subdivisions [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		1		1
Ending balance		1		1
Total Asset-Backed [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	184	279	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	4	(1)	7	(6)
Purchases		41	20	71
Sales				(72)
Settlements	(4)	(6)	(8)	(12)
Transfers into Level 3	40	13	45	13
Transfers out of Level 3	(31)	(53)	(68)	(63)
Ending balance	193	273	193	273
Unrealized Gains (Losses) Recognized in other Comprehensive Income (Loss) on Level 3 Assets and Liabilities	5		8
Total Fixed Maturities Available-for-Sale [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	437	380	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	16	(2)	27	(7)
Purchases	76	43	152	73
Sales		(5)		(77)
Settlements	(6)	(8)	(12)	(16)
Transfers into Level 3	40	13	45	18
Transfers out of Level 3	(32)	(53)	(100)	(63)
Ending balance	531	368	531	368
Unrealized Gains (Losses) Recognized in Net Income (Loss) on Level 3 Assets and Liabilities		0		0
Unrealized Gains (Losses) Recognized in other Comprehensive Income (Loss) on Level 3 Assets and Liabilities	15		25
Fixed Maturities Trading [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	5	7	6	4
Net Realized Investment Gains (Losses) and Net Change in Unrealized Investment Gains (Losses), Included in Net Income (Loss)	(1)		(2)	3
Ending balance	4	7	4	7
Unrealized Gains (Losses) Recognized in Net Income (Loss) on Level 3 Assets and Liabilities	(1)		(2)	3
Fixed Maturity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	442	387	425	438
Net Realized Investment Gains (Losses) and Net Change in Unrealized Investment Gains (Losses), Included in Net Income (Loss)	(1)	0	(2)	9
Net Realized Investment Gains (Losses) and Net Change in Unrealized Investment Gains (Losses), Included in OCI	16	(2)	27	(7)
Purchases	76	43	152	73
Sales		(5)		(77)
Settlements	(6)	(8)	(12)	(16)
Transfers into Level 3	40	13	45	18
Transfers out of Level 3	(32)	(53)	(100)	(63)
Ending balance	535	375	535	375
Unrealized Gains (Losses) Recognized in Net Income (Loss) on Level 3 Assets and Liabilities	(1)	0	(2)	3
Unrealized Gains (Losses) Recognized in other Comprehensive Income (Loss) on Level 3 Assets and Liabilities	15		25
Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	21	20	19	22
Net Realized Investment Gains (Losses) and Net Change in Unrealized Investment Gains (Losses), Included in Net Income (Loss)		(1)	2	(3)
Purchases	2		2
Sales		(1)		(1)
Ending balance	$ 23	18	23	18
Unrealized Gains (Losses) Recognized in Net Income (Loss) on Level 3 Assets and Liabilities		$ (1)	$ 3	$ (3)

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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-Section 50
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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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