Investments - Summary of Cost and Carrying Value of Equity and Trading Debt Securities (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Total trading debt securities, Cost	$ 1,926	$ 1,231
Equity securities at FVTNI, Cost	2,055	1,769
Total trading debt, Carrying Value	1,964	1,249
Equity securities at FVTNI, Carrying Value	2,317	1,926
Corporate Debt [Member]
Schedule of Investments [Line Items]
Total trading debt securities, Cost	1,591	822
Total trading debt, Carrying Value	1,641	844
Government Debt [Member]
Schedule of Investments [Line Items]
Total trading debt securities, Cost	203	268
Total trading debt, Carrying Value	210	269
Asset/Mortgage-Backed Debt [Member]
Schedule of Investments [Line Items]
Total trading debt securities, Cost	132	141
Total trading debt, Carrying Value	113	136
Equity Securities/Mutual Funds [Member]
Schedule of Investments [Line Items]
Equity securities at FVTNI, Cost	2,055	1,769
Equity securities at FVTNI, Carrying Value	$ 2,317	$ 1,926

Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=99377092&loc=SL75136599-209740

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Amount of investment in debt security measured at fair value with change in fair value recognized in net income (trading).

+ References

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=121553693&loc=d3e26610-111562

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-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121553693&loc=d3e26610-111562

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