Regulatory Capital - Summary of Regulatory Capital Requirement (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total capital (to risk-weighted assets), Actual, Amount	$ 509,214	$ 427,616
Total capital (to risk-weighted assets), Actual, Ratio	13.25%	13.39%
Total capital (to risk-weighted assets), For capital adequacy purposes, Amount	$ 307,362	$ 255,419
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	$ 403,412	$ 315,283
Total capital (to risk-weighted assets), Required for capital1 adequacy purposes with capital buffer, Ratio	10.50%	9.875%
Tier 1 capital (to average assets), Actual, Amount	$ 491,302	$ 409,760
Tier 1 capital (to average assets), Actual, Ratio	12.79%	12.83%
Tier 1 capital (to average assets), For capital adequacy purposes, Amount	$ 230,521	$ 191,565
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	$ 326,572	$ 251,429
Tier 1 capital (to average assets), Required for capital1 adequacy purposes with capital buffer, Ratio	8.50%	7.875%
Common equity tier 1 capital, Actual Amount	$ 433,809	$ 371,297
Common equity tier 1 capital, Actual Ratio	11.29%	11.63%
Common equity tier 1 capital,For capital adequacy purposes, Amount	$ 172,890	$ 143,673
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	$ 268,941	$ 203,537
Common equity tier 1 capital (to risk-weighted assets), Required for capital1 adequacy purposes with capital buffer, Ratio	7.00%	6.375%
Tier 1 capital (to average assets), Actual, Amount	$ 491,302	$ 409,760
Tier 1 capital (to average assets), Actual, Ratio	11.85%	10.12%
Tier 1 capital (to average assets), For capital adequacy purposes, Amount	$ 165,789	$ 162,033
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	$ 165,789	$ 162,033
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	$ 472,621	$ 396,755
Total capital (to risk-weighted assets), Actual, Ratio	12.30%	12.43%
Total capital (to risk-weighted assets), For capital adequacy purposes, Amount	$ 307,361	$ 255,419
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	$ 403,411	$ 315,283
Total capital (to risk-weighted assets), Required for capital1 adequacy purposes with capital buffer, Ratio	10.50%	9.875%
Total capital (to risk-weighted assets), For well capitalized purpose, Amount	$ 384,201	$ 319,274
Total capital (to risk-weighted assets), For well capitalized purpose, Ratio	10.00%	10.00%
Tier 1 capital (to average assets), Actual, Amount	$ 454,709	$ 378,899
Tier 1 capital (to average assets), Actual, Ratio	11.84%	11.87%
Tier 1 capital (to average assets), For capital adequacy purposes, Amount	$ 230,520	$ 191,565
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	$ 326,570	$ 251,429
Tier 1 capital (to average assets), Required for capital1 adequacy purposes with capital buffer, Ratio	8.50%	7.875%
Tier 1 capital (to average assets), For well capitalized purpose, Amount	$ 307,360	$ 255,420
Tier 1 capital (to average assets), For well capitalized purposes, Ratio	8.00%	8.00%
Common equity tier 1 capital, Actual Amount	$ 454,709	$ 378,899
Common equity tier 1 capital, Actual Ratio	11.84%	11.87%
Common equity tier 1 capital,For capital adequacy purposes, Amount	$ 172,890	$ 143,674
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	$ 268,940	$ 203,537
Common equity tier 1 capital (to risk-weighted assets), Required for capital1 adequacy purposes with capital buffer, Ratio	7.00%	6.375%
Common equity tier 1 capital, For well capitalized purpose, Amount	$ 249,730	$ 207,528
Common equity tier 1 capital, For well capitalized purposes, Ratio	6.50%	6.50%
Tier 1 capital (to average assets), Actual, Amount	$ 454,709	$ 378,899
Tier 1 capital (to average assets), Actual, Ratio	10.99%	9.34%
Tier 1 capital (to average assets), For capital adequacy purposes, Amount	$ 165,523	$ 162,327
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	$ 165,523	$ 162,327
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	$ 206,904	$ 202,908
Tier 1 capital (to average assets), For well capitalized purposes, Ratio	5.00%	5.00%