Securities - Fair Value and Amortized Cost of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Investments Debt And Equity Securities [Abstract]
Available-for-Sale, Amortized Cost, Mortgage-backed securities	$ 186,157
Available-for-Sale, Amortized Cost, Total debt securities	186,157
Available-for-Sale, Fair Value, Mortgage-backed securities	180,238
Available-for-Sale, Fair Value, Total debt securities	180,238
Held-to-Maturity, Amortized Cost, Within one year	5,055
Held-to-Maturity, Amortized Cost, One to five years	35,190
Held-to-Maturity, Amortized Cost, Five to ten years	56,830
Held-to-Maturity, Amortized Cost, After ten years	87,626
Held-to-Maturity, Amortized Cost, Mortgage-backed securities	481,294
Held-to-Maturity, Amortized Cost	665,995	$ 535,462
Held-to-Maturity, Fair Value, Within one year	5,057
Held-to-Maturity, Fair Value, One to five years	35,342
Held-to-Maturity, Fair Value, Five to ten years	57,288
Held-to-Maturity, Fair Value, After ten years	86,297
Held-to-Maturity, Fair Value, Mortgage-backed securities	469,028
Held-to-Maturity, Fair Value, Total debt securities	$ 653,012	$ 532,744