Securities - Fair Value and Amortized Cost of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Investments Debt And Equity Securities [Abstract]
Available-for-Sale, Amortized Cost, Mortgage-backed securities	$ 152,068
Available-for-Sale, Amortized Cost	152,068	$ 173,503
Available-for-Sale, Fair Value, Mortgage-backed securities	152,680
Available-for-Sale, Fair Value, Total debt securities	152,680	168,875
Held-to-Maturity, Amortized Cost, Within one year	15,851
Held-to-Maturity, Amortized Cost, One to five years	33,321
Held-to-Maturity, Amortized Cost, Five to ten years	54,978
Held-to-Maturity, Amortized Cost, After ten years	75,783
Held-to-Maturity, Amortized Cost, Mortgage-backed securities	584,230
Held-to-Maturity, Amortized Cost	764,163	748,356
Held-to-Maturity, Fair Value, Within one year	15,888
Held-to-Maturity, Fair Value, One to five years	34,018
Held-to-Maturity, Fair Value, Five to ten years	56,338
Held-to-Maturity, Fair Value, After ten years	77,834
Held-to-Maturity, Fair Value, Mortgage-backed securities	594,888
Held-to-Maturity, Fair Value, Total debt securities	$ 778,966	$ 739,989