Regulatory Matters - Schedule of Regulatory Capital Ratios Under the Basel III Regulatory Capital Framework (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital to Risk-Weighted Assets, Actual Ratio	19.85%	18.45%
Tier1 Capital to Risk-Weighted Assets, Actual Ratio	18.66%	17.30%
Leverage Ratio, Actual Ratio	11.09%	10.71%
Consolidated [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital to Risk-Weighted Assets, Actual Amount	$ 814,634	$ 739,959
Tier1 Capital to Risk-Weighted Assets, Actual Amount	765,882	693,584
Common Equity Tier1 Capital to Risk-Weighted Assets, Actual Amount	765,882	693,584
Leverage Ratio, Actual Amount	$ 765,882	$ 693,584
Total Capital to Risk-Weighted Assets, Actual Ratio	19.85%	18.45%
Tier1 Capital to Risk-Weighted Assets, Actual Ratio	18.66%	17.30%
Common Equity Tier1 Capital to Risk-Weighted Assets, Actual Ratio	18.66%	17.30%
Leverage Ratio, Actual Ratio	11.09%	10.71%
Consolidated [Member] | Minimum [Member] | Basel III Fully Phased-In [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital to Risk-Weighted Assets, Minimum Capital Required	$ 430,872	$ 421,007
Tier1 Capital to Risk-Weighted Assets, Minimum Capital Required	348,801	340,815
Common Equity Tier1 Capital to Risk-Weighted Assets, Minimum Capital Required	287,248	280,671
Leverage Ratio, Minimum Capital Required	$ 276,296	$ 258,978
Total Capital to Risk-Weighted Assets, Minimum Capital Required	10.50%	10.50%
Tier1 Capital to Risk-Weighted Assets, Minimum Capital Required	8.50%	8.50%
Common Equity Tier1 Capital to Risk-Weighted Assets, Minimum Capital Required, Ratio	7.00%	7.00%
Leverage Ratio, Minimum Capital Required, Ratio	4.00%	4.00%
Consolidated [Member] | Minimum [Member] | Basel III Phase-In [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital to Risk-Weighted Assets, Minimum Capital Required	$ 379,578	$ 345,827
Tier1 Capital to Risk-Weighted Assets, Minimum Capital Required	297,507	265,635
Common Equity Tier1 Capital to Risk-Weighted Assets, Minimum Capital Required	235,954	205,491
Leverage Ratio, Minimum Capital Required	$ 276,296	$ 258,978
Total Capital to Risk-Weighted Assets, Minimum Capital Required	9.25%	8.625%
Tier1 Capital to Risk-Weighted Assets, Minimum Capital Required	7.25%	6.625%
Common Equity Tier1 Capital to Risk-Weighted Assets, Minimum Capital Required, Ratio	5.75%	5.125%
Leverage Ratio, Minimum Capital Required, Ratio	4.00%	4.00%
Parent Company [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital to Risk-Weighted Assets, Actual Amount	$ 723,563	$ 633,403
Tier1 Capital to Risk-Weighted Assets, Actual Amount	674,811	587,028
Common Equity Tier1 Capital to Risk-Weighted Assets, Actual Amount	674,811	587,028
Leverage Ratio, Actual Amount	$ 674,811	$ 587,028
Total Capital to Risk-Weighted Assets, Actual Ratio	17.68%	15.84%
Tier1 Capital to Risk-Weighted Assets, Actual Ratio	16.49%	14.68%
Common Equity Tier1 Capital to Risk-Weighted Assets, Actual Ratio	16.49%	14.68%
Leverage Ratio, Actual Ratio	9.80%	9.10%
Parent Company [Member] | Minimum [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital to Risk-Weighted Assets, Required To Be Considered Well Capitalized , Ratio	10.00%	10.00%
Total Capital to Risk-Weighted Assets, Required To Be Considered Well Capitalized, Amount	$ 409,312	$ 399,919
Tier1 Capital to Risk-Weighted Assets, Required To Be Considered Well Capitalized , Ratio	8.00%	8.00%
Tier1 Capital to Risk-Weighted Assets, Required To Be Considered Well Capitalized, Amount	$ 327,450	$ 319,935
Common Equity Tier1 Capital to Risk-Weighted Assets, Required To Be Considered Well Capitalized , Ratio	6.50%	6.50%
Common Equity Tier1 Capital to Risk-Weighted Assets, Required To Be Considered Well Capitalized, Amount	$ 266,053	$ 259,947
Leverage Ratio, Required To Be Considered Well Capitalized , Ratio	5.00%	5.00%
Leverage Ratio, Required To Be Considered Well Capitalized, Amount	$ 344,151	$ 322,426
Parent Company [Member] | Minimum [Member] | Basel III Fully Phased-In [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital to Risk-Weighted Assets, Minimum Capital Required	429,777	419,915
Tier1 Capital to Risk-Weighted Assets, Minimum Capital Required	347,915	339,931
Common Equity Tier1 Capital to Risk-Weighted Assets, Minimum Capital Required	286,518	279,943
Leverage Ratio, Minimum Capital Required	$ 275,320	$ 257,941
Total Capital to Risk-Weighted Assets, Minimum Capital Required	10.50%	10.50%
Tier1 Capital to Risk-Weighted Assets, Minimum Capital Required	8.50%	8.50%
Common Equity Tier1 Capital to Risk-Weighted Assets, Minimum Capital Required, Ratio	7.00%	7.00%
Leverage Ratio, Minimum Capital Required, Ratio	4.00%	4.00%
Parent Company [Member] | Minimum [Member] | Basel III Phase-In [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital to Risk-Weighted Assets, Minimum Capital Required	$ 378,614	$ 344,930
Tier1 Capital to Risk-Weighted Assets, Minimum Capital Required	296,751	264,946
Common Equity Tier1 Capital to Risk-Weighted Assets, Minimum Capital Required	235,354	204,959
Leverage Ratio, Minimum Capital Required	$ 275,320	$ 257,941
Total Capital to Risk-Weighted Assets, Minimum Capital Required	9.25%	8.625%
Tier1 Capital to Risk-Weighted Assets, Minimum Capital Required	7.25%	6.625%
Common Equity Tier1 Capital to Risk-Weighted Assets, Minimum Capital Required, Ratio	5.75%	5.125%
Leverage Ratio, Minimum Capital Required, Ratio	4.00%	4.00%