Regulatory Matters - Schedule of Regulatory Capital Ratios Under the Basel III Regulatory Capital Framework (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 24, 2018	Dec. 31, 2017
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital to Risk-Weighted Assets, Actual Ratio	20.61%		19.85%
Tier1 Capital to Risk-Weighted Assets, Actual Ratio	19.47%		18.66%
Leverage Ratio, Actual Ratio	11.85%		11.09%
Consolidated [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital to Risk-Weighted Assets, Actual Amount	$ 940,026		$ 814,634
Tier1 Capital to Risk-Weighted Assets, Actual Amount	888,015		765,882
Common Equity Tier1 Capital to Risk-Weighted Assets, Actual Amount	888,015		765,882
Leverage Ratio, Actual Amount	$ 888,015		$ 765,882
Total Capital to Risk-Weighted Assets, Actual Ratio	20.61%		19.85%
Tier1 Capital to Risk-Weighted Assets, Actual Ratio	19.47%		18.66%
Common Equity Tier1 Capital to Risk-Weighted Assets, Actual Ratio	19.47%		18.66%
Leverage Ratio, Actual Ratio	11.85%		11.09%
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	$ 478,969	$ 430,872
Tier1 Capital to Risk-Weighted Assets, Minimum Capital Required	387,737	348,801
Common Equity Tier1 Capital to Risk-Weighted Assets, Minimum Capital Required	319,312	287,248
Leverage Ratio, Minimum Capital Required	$ 299,682	276,296
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	$ 450,459		$ 379,578
Tier1 Capital to Risk-Weighted Assets, Minimum Capital Required	359,226		297,507
Common Equity Tier1 Capital to Risk-Weighted Assets, Minimum Capital Required	290,802		235,954
Leverage Ratio, Minimum Capital Required	$ 299,682		$ 276,296
Total Capital to Risk-Weighted Assets, Minimum Capital Required	9.875%		9.25%
Tier1 Capital to Risk-Weighted Assets, Minimum Capital Required	7.875%		7.25%
Common Equity Tier1 Capital to Risk-Weighted Assets, Minimum Capital Required, Ratio	6.375%		5.75%
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	$ 824,428		$ 723,563
Tier1 Capital to Risk-Weighted Assets, Actual Amount	772,417		674,811
Common Equity Tier1 Capital to Risk-Weighted Assets, Actual Amount	772,417		674,811
Leverage Ratio, Actual Amount	$ 772,417		$ 674,811
Total Capital to Risk-Weighted Assets, Actual Ratio	18.12%		17.68%
Tier1 Capital to Risk-Weighted Assets, Actual Ratio	16.97%		16.49%
Common Equity Tier1 Capital to Risk-Weighted Assets, Actual Ratio	16.97%		16.49%
Leverage Ratio, Actual Ratio	10.35%		9.80%
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	$ 455,038		$ 409,312
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	$ 364,030		$ 327,450
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	$ 295,775		$ 266,053
Leverage Ratio, Required To Be Considered Well Capitalized , Ratio	5.00%		5.00%
Leverage Ratio, Required To Be Considered Well Capitalized, Amount	$ 373,220		$ 344,151
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	477,790	429,777
Tier1 Capital to Risk-Weighted Assets, Minimum Capital Required	386,782	347,915
Common Equity Tier1 Capital to Risk-Weighted Assets, Minimum Capital Required	318,526	286,518
Leverage Ratio, Minimum Capital Required	$ 298,576	$ 275,320
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	$ 449,350		$ 378,614
Tier1 Capital to Risk-Weighted Assets, Minimum Capital Required	358,342		296,751
Common Equity Tier1 Capital to Risk-Weighted Assets, Minimum Capital Required	290,087		235,354
Leverage Ratio, Minimum Capital Required	$ 298,576		$ 275,320
Total Capital to Risk-Weighted Assets, Minimum Capital Required	9.875%		9.25%
Tier1 Capital to Risk-Weighted Assets, Minimum Capital Required	7.875%		7.25%
Common Equity Tier1 Capital to Risk-Weighted Assets, Minimum Capital Required, Ratio	6.375%		5.75%
Leverage Ratio, Minimum Capital Required, Ratio	4.00%		4.00%