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Loans and the Allowance for Credit Losses - Allowance for Credit Losses and Recorded Investments in Loans (Parenthetical) (Detail) - USD ($) $ in Millions | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2019 | Dec. 31, 2018
Receivables [Abstract] |  |
Troubled debt restructuring threshold | $ 1 | $ 1

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- References No definition available. + Details
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- Definition Threshold for individually evaluating commercial loans for impairment. + References No definition available. + Details
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