Regulatory Capital - Summary of Capital Amounts, Tier 1 Risk Based and Tier 1 Leveraged Ratios (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Jan. 01, 2018	Dec. 31, 2017	Jan. 01, 2017
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common equity Tier 1 capital (to risk weighted assets), Actual Amount	$ 740,968		$ 689,494
Tier 1 capital (to risk weighted assets), Actual Amount	958,418		906,963
Total capital (to risk weighted assets), Actual Amount	1,080,245		1,021,601
Tier 1 capital (to average assets), Actual Amount	$ 958,418		$ 906,963
Common equity Tier 1 (to risk weighted assets), Actual Ratio	8.703%		8.805%
Tier 1 capital (to risk weighted assets), Actual Ratio	11.257%		11.583%
Total capital (to risk weighted assets), Actual Ratio	12.688%		13.047%
Tier 1 capital (to average assets), Actual Ratio	8.913%		8.937%
Common equity Tier 1 capital (to risk weighted assets), For Capital Adequacy Purposes Amount	$ 383,113		$ 352,368
Tier 1 capital (to risk weighted assets), For Capital Adequacy Purposes Amount	510,818		469,824
Total capital (to risk weighted assets), For Capital Adequacy Purposes Amount	681,090		626,432
Tier 1 capital (to average assets), For Capital Adequacy Purposes Amount	$ 430,099		$ 405,949
Common equity Tier 1 (to risk weighted assets), For Capital Adequacy Purposes Ratio	4.50%	6.375%	4.50%
Tier 1 capital (to risk weighted assets), For Capital Adequacy Purposes Ratio	6.00%	7.875%	6.00%
Total capital (to risk weighted assets), For Capital Adequacy Purposes Ratio	8.00%	9.875%	8.00%
Tier 1 capital (to average assets), For Capital Adequacy Purposes Ratio	4.00%		4.00%
Common equity Tier 1 (to risk weighted assets), for Basel III amount	$ 542,744		$ 450,248
Tier 1 (to risk weighted assets) Required for Basel III amount	670,448		567,704
Total capital (to risk weighted assets), for Basel III amount	840,721		724,313
Tier 1 (to risk average assets), for Basel III amount	$ 430,099		$ 405,949
Common equity Tier 1 (to risk weighted assets), for Basel III ratio	6.375%		5.75%
Tier 1 capital (to risk weighted assets), for Basel III ratio	7.875%		7.25%
Total capital (to risk weighted assets), for Basel III ratio	9.875%		9.25%
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	$ 1,054,869		$ 1,023,564
Tier 1 capital (to risk weighted assets), Actual Amount	1,054,869		1,023,564
Total capital (to risk weighted assets), Actual Amount	1,204,825		1,170,666
Tier 1 capital (to average assets), Actual Amount	$ 1,054,869		$ 1,023,564
Common equity Tier 1 (to risk weighted assets), Actual Ratio	12.393%		13.081%
Tier 1 capital (to risk weighted assets), Actual Ratio	12.393%		13.081%
Total capital (to risk weighted assets), Actual Ratio	14.154%		14.961%
Tier 1 capital (to average assets), Actual Ratio	9.814%		10.092%
Common equity Tier 1 capital (to risk weighted assets), For Capital Adequacy Purposes Amount	$ 383,042		$ 352,122
Tier 1 capital (to risk weighted assets), For Capital Adequacy Purposes Amount	510,722		469,496
Total capital (to risk weighted assets), For Capital Adequacy Purposes Amount	680,963		625,994
Tier 1 capital (to average assets), For Capital Adequacy Purposes Amount	$ 429,939		$ 405,701
Common equity Tier 1 (to risk weighted assets), For Capital Adequacy Purposes Ratio	4.50%		4.50%	6.375%
Tier 1 capital (to risk weighted assets), For Capital Adequacy Purposes Ratio	6.00%		6.00%	7.875%
Total capital (to risk weighted assets), For Capital Adequacy Purposes Ratio	8.00%		8.00%	9.875%
Tier 1 capital (to average assets), For Capital Adequacy Purposes Ratio	4.00%		4.00%
Common equity Tier 1 Capital (to risk weighted assets), To Be Well Capitalized Under Prompt Corrective Action Provisions Amount	$ 553,282		$ 508,621
Tier 1 capital (to risk weighted assets), To Be Well Capitalized Under Prompt Corrective Action Provisions Amount	680,963		625,994
Total capital (to risk weighted assets), To Be Well Capitalized Under Prompt Corrective Action Provisions Amount	851,204		782,493
Tier 1 capital (to average assets), To Be Well Capitalized Under Prompt Corrective Action Provisions Amount	$ 537,423		$ 507,126
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	8.00%		8.00%
Total capital (to risk weighted assets), To Be Well Capitalized Under Prompt Corrective Action Provisions Ratio	10.00%		10.00%
Tier 1 capital (to average assets), To Be Well Capitalized Under Prompt Corrective Action Provisions Ratio	5.00%		5.00%
Common equity Tier 1 (to risk weighted assets), for Basel III amount	$ 542,642		$ 449,934
Tier 1 (to risk weighted assets) Required for Basel III amount	670,323		567,307
Total capital (to risk weighted assets), for Basel III amount	840,563		723,806
Tier 1 (to risk average assets), for Basel III amount	$ 429,939		$ 405,701
Common equity Tier 1 (to risk weighted assets), for Basel III ratio	6.375%		5.75%
Tier 1 capital (to risk weighted assets), for Basel III ratio	7.875%		7.25%
Total capital (to risk weighted assets), for Basel III ratio	9.875%		9.25%
Tier 1 capital (to average assets), for Basel III ratio	4.00%		4.00%