Fair Value - Quantitative Information about Significant Unobservable Inputs Utilized by Company in Fair Value Measurements of Level 3 Assets (Detail) $ in Millions	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated fair value	$ 39,778	$ 41,484
Level 3 [Member] \| Fixed Maturity Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated fair value	$ 130	$ 136
Minimum [Member] \| Level 3 [Member] \| Credit Spread [Member] \| Discounted Cash Flow [Member] \| Fixed Maturity Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Measurement input	0.01	0.01
Maximum [Member] \| Level 3 [Member] \| Credit Spread [Member] \| Discounted Cash Flow [Member] \| Fixed Maturity Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Measurement input	0.12	0.12
Weighted Average [Member] \| Level 3 [Member] \| Credit Spread [Member] \| Discounted Cash Flow [Member] \| Fixed Maturity Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Measurement input	0.03	0.03

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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- Definition

Value of input used to measure investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

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+ References

No definition available.

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