Investment Securities - Amount of Gross Gains and Losses Realized through Sales of Available-for-Sale Investment Securities (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Available-for-sale Securities, Gross Realized Gain (Loss), Excluding Other than Temporary Impairments [Abstract]
Realized gains	$ 10	$ 9	$ 25	$ 65
Realized losses				(18)
Net realized gains (losses)	10	9	25	47
Income tax (benefit) on net realized gains (losses)	$ 2	$ 3	$ 6	$ 18

This item represents the gross profit realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27357-111563

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- 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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26805-111562

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- References

No definition available.

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- Definition

Amount of loss realized on the sale of debt or equity securities classified as available-for-sale. Excludes other than temporary impairments (OTTI).

+ References

No definition available.

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- Definition

Available for sale securities gross realized gain (loss) excluding other than temporary impairments income tax expense (benefit).

+ References

No definition available.

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