Securities - Schedule of Fair Value of Available-for-Sale and Held-to-Maturity Debt Securities by Contractual Maturity (Parenthetical) (Detail) - USD ($) $ in Thousands |
Mar. 31, 2018 |
Dec. 31, 2017 |
|---|---|---|
| Amortized Cost and Fair Value Debt Securities [Abstract] | ||
| Held-to-maturity, Amortized Cost | $ 1,006,042 | $ 1,009,500 |