Regulatory Capital - Summary of Capital Amounts, Tier 1 Risk Based and Tier 1 Leveraged Ratios (Detail) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common equity Tier 1 capital (to risk weighted assets), Actual Amount	$ 853,818	$ 821,810
Tier 1 capital (to risk weighted assets), Actual Amount	1,071,289	1,039,281
Total capital (to risk weighted assets), Actual Amount	1,311,527	1,256,309
Tier 1 capital (to average assets), Actual Amount	$ 1,071,289	$ 1,039,281
Common equity Tier 1 (to risk weighted assets), Actual Ratio	7.765%	7.984%
Tier 1 capital (to risk weighted assets), Actual Ratio	0.09743	0.10096
Total capital (to risk weighted assets), Actual Ratio	0.11928	0.12205
Tier 1 capital (to average assets), Actual Ratio	0.08790	0.09258
Common equity Tier 1 capital (to risk weighted assets), For Capital Adequacy Purposes Amount	$ 494,780	$ 463,211
Tier 1 capital (to risk weighted assets), For Capital Adequacy Purposes Amount	659,706	617,615
Total capital (to risk weighted assets), For Capital Adequacy Purposes Amount	879,608	823,487
Tier 1 capital (to average assets), For Capital Adequacy Purposes Amount	$ 487,512	$ 449,026
Common equity Tier 1 (to risk weighted assets), For Capital Adequacy Purposes Ratio	4.50%	4.50%
Tier 1 capital (to risk weighted assets), For Capital Adequacy Purposes Ratio	0.06000	0.06000
Total capital (to risk weighted assets), For Capital Adequacy Purposes Ratio	0.08000	0.08000
Tier 1 capital (to average assets), For Capital Adequacy Purposes Ratio	0.04000	0.04000
Common equity Tier 1 (to risk weighted assets), for Basel III amount	$ 769,657	$ 720,551
Tier 1 (to risk weighted assets) Required for Basel III amount	934,584	874,955
Total capital (to risk weighted assets), for Basel III amount	1,154,486	1,080,827
Tier 1 (to risk average assets), for Basel III amount	$ 487,512	$ 449,026
Common equity Tier 1 (to risk weighted assets), for Basel III ratio	7.00%	7.00%
Tier 1 capital (to risk weighted assets), for Basel III ratio	8.50%	8.50%
Total capital (to risk weighted assets), for Basel III ratio	10.50%	10.50%
Tier 1 capital (to average assets), for Basel III ratio	4.00%	4.00%
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Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common equity Tier 1 capital (to risk weighted assets), Actual Amount	$ 1,168,276	$ 1,164,652
Tier 1 capital (to risk weighted assets), Actual Amount	1,168,276	1,164,652
Total capital (to risk weighted assets), Actual Amount	1,351,665	1,330,155
Tier 1 capital (to average assets), Actual Amount	$ 1,168,276	$ 1,164,652
Common equity Tier 1 (to risk weighted assets), Actual Ratio	10.636%	11.323%
Tier 1 capital (to risk weighted assets), Actual Ratio	0.10636	0.11323
Total capital (to risk weighted assets), Actual Ratio	0.12305	0.12933
Tier 1 capital (to average assets), Actual Ratio	0.09593	0.10379
Common equity Tier 1 capital (to risk weighted assets), For Capital Adequacy Purposes Amount	$ 494,291	$ 462,842
Tier 1 capital (to risk weighted assets), For Capital Adequacy Purposes Amount	659,055	617,122
Total capital (to risk weighted assets), For Capital Adequacy Purposes Amount	878,740	822,829
Tier 1 capital (to average assets), For Capital Adequacy Purposes Amount	$ 487,151	$ 448,851
Common equity Tier 1 (to risk weighted assets), For Capital Adequacy Purposes Ratio	4.50%	4.50%
Tier 1 capital (to risk weighted assets), For Capital Adequacy Purposes Ratio	0.06000	0.06000
Total capital (to risk weighted assets), For Capital Adequacy Purposes Ratio	0.08000	0.08000
Tier 1 capital (to average assets), For Capital Adequacy Purposes Ratio	0.04000	0.04000
Common equity Tier 1 Capital (to risk weighted assets), To Be Well Capitalized Under Prompt Corrective Action Provisions Amount	$ 713,976	$ 668,549
Tier 1 capital (to risk weighted assets), To Be Well Capitalized Under Prompt Corrective Action Provisions Amount	878,740	822,829
Total capital (to risk weighted assets), To Be Well Capitalized Under Prompt Corrective Action Provisions Amount	1,098,424	1,028,537
Tier 1 capital (to average assets), To Be Well Capitalized Under Prompt Corrective Action Provisions Amount	$ 608,938	$ 561,064
Common equity Tier 1 (to risk weighted assets), To Be Well Capitalized Under Prompt Corrective Action Provisions Ratio	6.50%	6.50%
Tier 1 capital (to risk weighted assets), To Be Well Capitalized Under Prompt Corrective Action Provisions Ratio	0.08000	0.08000
Total capital (to risk weighted assets), To Be Well Capitalized Under Prompt Corrective Action Provisions Ratio	0.10000	0.10000
Tier 1 capital (to average assets), To Be Well Capitalized Under Prompt Corrective Action Provisions Ratio	0.05000	0.05000
Common equity Tier 1 (to risk weighted assets), for Basel III amount	$ 768,897	$ 719,976
Tier 1 (to risk weighted assets) Required for Basel III amount	933,661	874,256
Total capital (to risk weighted assets), for Basel III amount	1,153,346	1,079,964
Tier 1 (to risk average assets), for Basel III amount	$ 487,151	$ 448,851
Common equity Tier 1 (to risk weighted assets), for Basel III ratio	7.00%	7.00%
Tier 1 capital (to risk weighted assets), for Basel III ratio	8.50%	8.50%
Total capital (to risk weighted assets), for Basel III ratio	10.50%	10.50%
Tier 1 capital (to average assets), for Basel III ratio	4.00%	4.00%