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, 2018	Mar. 31, 2017
Corporate Bonds and Other [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	$ 98	$ 130
Net Realized Gains (Losses) and Net Change in Unrealized Gains (Losses), Included in Net Income	(1)
Net Realized Gains (Losses) and Net Change in Unrealized Gains (Losses), Included in OCI		1
Purchases		5
Sales		(1)
Settlements	(2)	(14)
Transfers into Level 3	5
Ending balance	100	121
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	335	199
Net Realized Gains (Losses) and Net Change in Unrealized Gains (Losses), Included in Net Income	7
Net Realized Gains (Losses) and Net Change in Unrealized Gains (Losses), Included in OCI	(5)	2
Purchases	30	38
Sales	(72)
Settlements	(6)	(6)
Transfers into Level 3		28
Transfers out of Level 3	(10)	(5)
Ending balance	279	256
Total Fixed Maturities Available-for-Sale [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	434	330
Net Realized Gains (Losses) and Net Change in Unrealized Gains (Losses), Included in Net Income	6
Net Realized Gains (Losses) and Net Change in Unrealized Gains (Losses), Included in OCI	(5)	3
Purchases	30	43
Sales	(72)	(1)
Settlements	(8)	(20)
Transfers into Level 3	5	28
Transfers out of Level 3	(10)	(5)
Ending balance	380	378
Fixed Maturities Trading [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	4	6
Net Realized Gains (Losses) and Net Change in Unrealized Gains (Losses), Included in Net Income	3	(1)
Ending balance	7	5
Unrealized Gains (Losses) Recognized in Net Income on Level 3 Assets Held at December 31	3
Fixed Maturity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	438	336
Net Realized Gains (Losses) and Net Change in Unrealized Gains (Losses), Included in Net Income	9	(1)
Net Realized Gains (Losses) and Net Change in Unrealized Gains (Losses), Included in OCI	(5)	3
Purchases	30	43
Sales	(72)	(1)
Settlements	(8)	(20)
Transfers into Level 3	5	28
Transfers out of Level 3	(10)	(5)
Ending balance	387	383
Unrealized Gains (Losses) Recognized in Net Income on Level 3 Assets Held at December 31	3
Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	22	20
Net Realized Gains (Losses) and Net Change in Unrealized Gains (Losses), Included in Net Income	(2)
Net Realized Gains (Losses) and Net Change in Unrealized Gains (Losses), Included in OCI		1
Purchases		1
Sales		(2)
Ending balance	20	20
Unrealized Gains (Losses) Recognized in Net Income on Level 3 Assets Held at December 31	$ (2)
Equity Securities Available-for-Sale [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		19
Net Realized Gains (Losses) and Net Change in Unrealized Gains (Losses), Included in OCI		1
Purchases		1
Sales		(2)
Ending balance		19
Equity Securities Trading [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		1
Ending balance		1
Life Settlement Contracts [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		58
Net Realized Gains (Losses) and Net Change in Unrealized Gains (Losses), Included in Net Income		6
Sales		(13)
Settlements		(5)
Ending balance		46
Derivative Instruments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative financial instruments, net, Beginning balance		0
Derivative financial instruments, net, Net Realized Gains (Losses) and Net Change in Unrealized Gains (Losses), Included in Net Income		1
Derivative financial instruments, net, Net Realized Gains (Losses) and Net Change in Unrealized Gains (Losses), Included in OCI		0
Derivative financial instruments, net, Purchases		0
Derivative financial instruments, net, Sales		(1)
Derivative financial instruments, net, Settlements		0
Derivative financial instruments, net, Transfers into Level 3		0
Derivative financial instruments, net, Transfers out of Level 3		0
Derivative financial instruments, net, Ending balance		0
Unrealized Gains (Losses) Recognized in Net Income on Level 3 Liabilities Held at December 31		$ 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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Amount of unrealized (holding) gain (loss) which is included in the statement of income (or changes in net assets) related to those liabilities 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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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 the income statement of financial instrument classified as a derivative asset (liability) after deduction of derivative liability (asset), measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
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Amount of gain (loss) recognized in other comprehensive income (loss) of financial instrument classified as a derivative asset (liability) after deduction of derivative liability (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 a derivative asset (liability) after deduction of derivative liability (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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Amount of sales of financial instrument classified as a derivative asset (liability) after deduction of derivative liability (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 settlements of financial instrument classified as a derivative asset (liability) after deduction of derivative liability, measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Amount of transfers of financial instrument classified as a derivative asset (liability) after deduction of derivative liability (asset) into level 3 of the fair value hierarchy.

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Amount of transfers of financial instrument classified as derivative asset (liability) after deduction of derivative liability (asset) out of level 3 of the fair value hierarchy.

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Fair value of financial instrument classified as derivative asset (liability) after deduction of derivative liability (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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