Regulatory Capital - Summary of Regulatory Capital Requirement (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total capital (to risk-weighted assets), Actual, Amount		$ 384,800
Total capital (to risk-weighted assets), Actual, Ratio	0.00%	12.91%
Total capital (to risk-weighted assets), For capital adequacy purposes, Amount		$ 238,543
Total capital (to risk-weighted assets), For capital adequacy purposes, Ratio	8.00%	8.00%
Total capital (to risk-weighted assets), Required for capital1 adequacy purposes with capital buffer, Amount		$ 275,816
Total capital (to risk-weighted assets), Required for capital1 adequacy purposes with capital buffer, Ratio	9.25%	9.25%
Tier 1 capital (to average assets), Actual, Amount		$ 368,355
Tier 1 capital (to average assets), Actual, Ratio	0.00%	12.35%
Tier 1 capital (to average assets), For capital adequacy purposes, Amount		$ 178,907
Tier 1 capital (to average assets), For capital adequacy purpose, Ratio	6.00%	6.00%
Tier 1 capital (to average assets), Required for capital1 adequacy purposes with capital buffer, Amount		$ 216,180
Tier 1 capital (to average assets), Required for capital1 adequacy purposes with capital buffer, Ratio	7.25%	7.25%
Common equity tier 1 capital, Actual Amount		$ 329,892
Common equity tier 1 capital, Actual Ratio	0.00%	11.06%
Common equity tier 1 capital,For capital adequacy purposes, Amount		$ 134,181
Common equity tier 1 capital, For capital adequacy purpose, Ratio	4.50%	4.50%
Common equity tier 1 capital (to risk-weighted assets), Required for capital1 adequacy purposes with capital buffer, Amount		$ 171,454
Common equity tier 1 capital (to risk-weighted assets), Required for capital1 adequacy purposes with capital buffer, Ratio	5.75%	5.75%
Tier 1 capital (to average assets), Actual, Amount		$ 368,355
Tier 1 capital (to average assets), Actual, Ratio	0.00%	9.92%
Tier 1 capital (to average assets), For capital adequacy purposes, Amount		$ 148,503
Tier 1 capital (to average assets), For capital adequacy purpose, Ratio	4.00%	4.00%
Tier 1 capital (to average assets), Required for capital1 adequacy purposes with capital buffer, Amount		$ 148,503
Tier 1 capital (to average assets), Required for capital1 adequacy purposes with capital buffer, Ratio	4.00%	4.00%
Bank [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total capital (to risk-weighted assets), Actual, Amount	$ 390,855	$ 382,788
Total capital (to risk-weighted assets), Actual, Ratio	13.08%	12.85%
Total capital (to risk-weighted assets), For capital adequacy purposes, Amount	$ 239,082	$ 238,386
Total capital (to risk-weighted assets), For capital adequacy purposes, Ratio	8.00%	8.00%
Total capital (to risk-weighted assets), Required for capital1 adequacy purposes with capital buffer, Amount	$ 276,439	$ 275,634
Total capital (to risk-weighted assets), Required for capital1 adequacy purposes with capital buffer, Ratio	9.25%	9.25%
Total capital (to risk-weighted assets), For well capitalized purpose, Amount	$ 298,866	$ 297,982
Total capital (to risk-weighted assets), For well capitalized purpose, Ratio	10.00%	10.00%
Tier 1 capital (to average assets), Actual, Amount	$ 374,322	$ 366,343
Tier 1 capital (to average assets), Actual, Ratio	12.53%	12.29%
Tier 1 capital (to average assets), For capital adequacy purposes, Amount	$ 179,312	$ 178,790
Tier 1 capital (to average assets), For capital adequacy purpose, Ratio	6.00%	6.00%
Tier 1 capital (to average assets), Required for capital1 adequacy purposes with capital buffer, Amount	$ 216,668	$ 216,038
Tier 1 capital (to average assets), Required for capital1 adequacy purposes with capital buffer, Ratio	7.25%	7.25%
Tier 1 capital (to average assets), For well capitalized purpose, Amount	$ 239,093	$ 238,386
Tier 1 capital (to average assets), For well capitalized purposes, Ratio	8.00%	8.00%
Common equity tier 1 capital, Actual Amount	$ 335,859	$ 366,343
Common equity tier 1 capital, Actual Ratio	11.24%	12.29%
Common equity tier 1 capital,For capital adequacy purposes, Amount	$ 134,483	$ 134,092
Common equity tier 1 capital, For capital adequacy purpose, Ratio	4.50%	4.50%
Common equity tier 1 capital (to risk-weighted assets), Required for capital1 adequacy purposes with capital buffer, Amount	$ 171,840	$ 171,340
Common equity tier 1 capital (to risk-weighted assets), Required for capital1 adequacy purposes with capital buffer, Ratio	5.75%	5.75%
Common equity tier 1 capital, For well capitalized purpose, Amount	$ 194,263	$ 193,689
Common equity tier 1 capital, For well capitalized purposes, Ratio	6.50%	6.50%
Tier 1 capital (to average assets), Actual, Amount	$ 374,322	$ 366,343
Tier 1 capital (to average assets), Actual, Ratio	9.82%	9.89%
Tier 1 capital (to average assets), For capital adequacy purposes, Amount	$ 152,494	$ 148,116
Tier 1 capital (to average assets), For capital adequacy purpose, Ratio	4.00%	4.00%
Tier 1 capital (to average assets), Required for capital1 adequacy purposes with capital buffer, Amount	$ 152,494	$ 148,116
Tier 1 capital (to average assets), Required for capital1 adequacy purposes with capital buffer, Ratio	4.00%	4.00%
Tier 1 capital (to average assets), For well capitalized purpose, Amount	$ 190,678	$ 185,145
Tier 1 capital (to average assets), For well capitalized purposes, Ratio	5.00%	5.00%