Investment Securities - Summary of Contractual Maturities of Investment Securities Classified as Available for Sale and Held to Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Available for Sale, Amortized Cost
Due in one year or less	$ 9,387
Due after one year through five years	109,655
Due after five years through ten years	56,380
Due after ten years	29,835
Total	205,257
Amortized Cost	650,473	$ 607,112
Available for Sale, Fair Value
Due in one year or less	9,387
Due after one year through five years	108,999
Due after five years through ten years	55,577
Due after ten years	29,752
Total	203,715
Total securities, Available for Sale, Fair Value	646,135	606,704
Held to Maturity, Amortized Cost
Due in one year or less	16,385
Due after one year through five years	38,986
Due after five years through ten years	23,568
Due after ten years	3,798
Total	82,737
Amortized Cost	139,685	147,614
Held to Maturity, Fair Value
Due in one year or less	16,401
Due after one year through five years	38,903
Due after five years through ten years	23,392
Due after ten years	3,795
Total	82,491
Total securities, Held to Maturity, Fair Value	138,688	146,990
Mortgage-backed securities
Available for Sale, Amortized Cost
Available for sale	429,671
Available for Sale, Fair Value
Available for sale	424,331
Held to Maturity, Amortized Cost
Held to maturity	56,948
Held to Maturity, Fair Value
Held to maturity	56,197
Equity securities
Available for Sale, Amortized Cost
Available for sale	15,545
Amortized Cost	15,545	17,314
Available for Sale, Fair Value
Available for sale	18,089
Total securities, Available for Sale, Fair Value	18,089	$ 21,882
Held to Maturity, Amortized Cost
Held to maturity	0
Held to Maturity, Fair Value
Held to maturity	$ 0