Investments - Securities Accounted for As Available-for-Sale Included in Investments (Parenthetical) (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Gross unrealized gain included in accumulated other comprehensive gain/(loss)	$ 724		$ 137
Available-for-sale securities sold	3,490	$ 8,080
Realized gains on sales of available-for-sale securities	$ 319	$ 131
Maximum [Member] \| Less than 5 years [Member]
Available For Sale Debt Securities Term	5 years
Maximum [Member] \| Greater than 5 years [Member]
Available For Sale Debt Securities Term	10 years
Minimum [Member] \| Less than 5 years [Member]
Available For Sale Debt Securities Term	1 year
Minimum [Member] \| Greater than 5 years [Member]
Available For Sale Debt Securities Term	5 years

Amount of investment sales in debt securities measured at fair value and accounted for as AFS.

No definition available.

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Balance Type:	debit
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- Definition

Amount of unrealized gain on 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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

+ Details

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Period Type:	duration

- Definition

This item represents the difference between the gross realized gains and losses realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities. Additionally, this item would include any losses recognized for other than temporary impairments (OTTI) of the subject investments in debt and equity securities.

+ References

No definition available.

+ Details

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- Definition

Period between issuance and maturity of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

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