Loans and the Allowance for Credit Losses - Allowance for Credit Losses and Recorded Investments in Loans (Parenthetical) (Detail) - USD ($) $ in Millions |
3 Months Ended | 12 Months Ended |
|---|---|---|
Mar. 31, 2018 |
Dec. 31, 2017 |
|
| Receivables [Abstract] | ||
| Troubled debt restructuring threshold | $ 1 | $ 1 |