Securities - Cumulative Credit Portion of Impairments on Debt Securities (Detail) $ in Millions	9 Months Ended
Sep. 30, 2018 USD ($)
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Cumulative Impairments on Debt Securities, Beginning Balance	$ 4
Impairment recognized in earnings	0
Cumulative Impairments on Debt Securities, Ending Balance	$ 4

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

No definition available.

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Balance Type:	na
Period Type:	duration