Securities - Cumulative Credit Portion of Impairments on Debt Securities (Details) $ in Millions	12 Months Ended
Dec. 31, 2017 USD ($)
Cumulative Impairments On Debt Securities [Roll Forward]
Cumulative Impairments on Debt Securities, Beginning Balance	$ 4
Impairment recognized in earnings	0
Cumulative Impairments on Debt Securities, Ending Balance	$ 4

Cumulative Impairments On Debt Securities [Roll Forward]

No definition available.

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