Securities - Fair Value and Amortized Cost of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Investments Debt And Equity Securities [Abstract]
Available-for-Sale, Amortized Cost, Mortgage-backed securities	$ 173,503
Available-for-Sale, Amortized Cost, Total debt securities	173,503
Available-for-Sale, Fair Value, Mortgage-backed securities	168,875
Available-for-Sale, Fair Value, Total debt securities	168,875
Held-to-Maturity, Amortized Cost, Within one year	6,425
Held-to-Maturity, Amortized Cost, One to five years	36,413
Held-to-Maturity, Amortized Cost, Five to ten years	54,205
Held-to-Maturity, Amortized Cost, After ten years	83,547
Held-to-Maturity, Amortized Cost, Mortgage-backed securities	567,766
Held-to-Maturity, Amortized Cost	748,356	$ 535,462
Held-to-Maturity, Fair Value, Within one year	6,474
Held-to-Maturity, Fair Value, One to five years	36,586
Held-to-Maturity, Fair Value, Five to ten years	54,275
Held-to-Maturity, Fair Value, After ten years	82,187
Held-to-Maturity, Fair Value, Mortgage-backed securities	560,467
Held-to-Maturity, Fair Value, Total debt securities	$ 739,989	$ 532,744