Investments - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Investments (Textual) [Abstract]
No mortgage loans past due and still accruing	90 days
Mortgage Loans	$ 0
Available-for-sale securities, carrying value	53,337	$ 50,280
Deposits [Member]
Investments (Textual) [Abstract]
Available-for-sale securities, carrying value	22	10
Available-for-sale securities, collateral value	$ 365	$ 88

Investments Textual [Abstract]

No definition available.

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- Definition

No mortgage loans past due and still accruing.

+ References

No definition available.

+ Details

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- Definition

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

No definition available.

+ Details

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- Definition

Amount of restricted 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 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (b)(1)(i)
-URI http://asc.fasb.org/extlink&oid=109249958&loc=SL6224234-111729

+ Details

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- Definition

Recorded investment in financing receivables that are 90 days or more past due and still accruing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5111-111524

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