Investment Securities - Summary of Contractual Maturities of Investment Securities Classified as Available for Sale and Held to Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Available for Sale, Amortized Cost
Due in one year or less	$ 4,825
Due after one year through five years	107,222
Due after five years through ten years	64,404
Due after ten years	22,963
Total	199,414
Total securities, Available for Sale, Amortized Cost	647,112	$ 607,112
Available for Sale, Fair Value
Due in one year or less	4,862
Due after one year through five years	107,526
Due after five years through ten years	63,961
Due after ten years	22,996
Total	199,345
Total securities, Available for Sale, Fair Value	647,292	606,704
Held to Maturity, Amortized Cost
Due in one year or less	18,662
Due after one year through five years	37,115
Due after five years through ten years	26,864
Due after ten years	3,807
Total	86,448
Amortized Cost	135,021	147,614
Held to Maturity, Fair Value
Due in one year or less	18,699
Due after one year through five years	37,324
Due after five years through ten years	26,957
Due after ten years	3,799
Total	86,779
Total securities, Held to Maturity, Fair Value	135,085	146,990
Mortgage Backed Securities [Member]
Available for Sale, Amortized Cost
Available for Sale, Amortized Cost	432,238
Available for Sale, Fair Value	430,135
Available for Sale, Fair Value
Available for Sale, Fair Value	430,135
Held to Maturity, Amortized Cost
Held to Maturity, Amortized Cost	48,573
Held to Maturity, Fair Value
Held to Maturity, Fair Value	48,306
Equity Securities [Member]
Available for Sale, Amortized Cost
Available for Sale, Amortized Cost	15,460
Total securities, Available for Sale, Amortized Cost	15,460	17,314
Available for Sale, Fair Value	17,812
Available for Sale, Fair Value
Available for Sale, Fair Value	17,812
Total securities, Available for Sale, Fair Value	$ 17,812	$ 21,882