Regulatory Matters - Regulatory Capital Amounts and Ratios for Company and Bank (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Company [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital (to Risk-Weighted Assets), Actual, Amount	$ 853,147	$ 801,719
Total Capital (to Risk-Weighted Assets), Actual, Ratio	18.24%	19.09%
Total Capital (to Risk-Weighted Assets), For Capital Adequacy Purposes, Amount	374,144	336,008
Tier 1 Capital (to Risk-Weighted Assets), Actual, Amount	794,576	748,825
Tier 1 Capital (to Risk-Weighted Assets), Actual, Ratio	16.99%	17.83%
Tier 1 Capital (to Risk-Weighted Assets), For Capital Adequacy Purposes, Amount	187,072	168,004
Tier 1 Capital (to Average-Assets), Actual, Amount	794,576	748,825
Tier 1 Capital (to Average-Assets), Actual, Ratio	10.86%	11.30%
Tier 1 Capital (to Average-Assets), For Capital Adequacy Purposes, Amount	292,615	265,035
Bank [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital (to Risk-Weighted Assets), Actual, Amount	845,951	794,239
Total Capital (to Risk-Weighted Assets), Actual, Ratio	18.11%	18.93%
Total Capital (to Risk-Weighted Assets), For Capital Adequacy Purposes, Amount	373,758	335,740
Total Capital (to Risk-Weighted Assets), Required To Be Well Capitalized, Amount	467,197	419,675
Tier 1 Capital (to Risk-Weighted Assets), Actual, Amount	787,439	741,386
Tier 1 Capital (to Risk-Weighted Assets), Actual, Ratio	16.85%	17.67%
Tier 1 Capital (to Risk-Weighted Assets), For Capital Adequacy Purposes, Amount	186,879	167,870
Tier 1 Capital (to Risk-Weighted Assets), Required To Be Well Capitalized, Amount	280,318	251,805
Tier 1 Capital (to Average-Assets), Actual, Amount	787,439	741,386
Tier 1 Capital (to Average-Assets), Actual, Ratio	10.77%	11.20%
Tier 1 Capital (to Average-Assets), For Capital Adequacy Purposes, Amount	292,392	264,893
Tier 1 Capital (to Average-Assets), Required To Be Well Capitalized, Amount	$ 365,490	$ 331,117
Minimum [Member] | Company [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital (to Risk-Weighted Assets), For Capital Adequacy Purposes, Ratio	8.00%	8.00%
Tier 1 Capital (to Risk-Weighted Assets), For Capital Adequacy Purposes, Ratio	4.00%	4.00%
Tier 1 Capital (to Average-Assets), For Capital Adequacy Purposes, Ratio	4.00%	4.00%
Minimum [Member] | Bank [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital (to Risk-Weighted Assets), For Capital Adequacy Purposes, Ratio	8.00%	8.00%
Total Capital (to Risk-Weighted Assets), Required To Be Well Capitalized, Ratio	10.00%	10.00%
Tier 1 Capital (to Risk-Weighted Assets), For Capital Adequacy Purposes, Ratio	4.00%	4.00%
Tier 1 Capital (to Risk-Weighted Assets), Required To Be Well Capitalized, Ratio	6.00%	6.00%
Tier 1 Capital (to Average-Assets), For Capital Adequacy Purposes, Ratio	4.00%	4.00%
Tier 1 Capital (to Average-Assets), Required To Be Well Capitalized, Ratio	5.00%	5.00%