Allowance for Loan Losses and Credit Quality Information - Summary of Nonaccrual and Past Due Loans (Detail) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	$ 8,934		$ 7,730
Accruing Current Balances	4,629,179		4,404,996
Acquired Impaired Loans	24,582	$ 38,103	33,614
Nonaccrual Loans	33,536		22,876
Total Loans	$ 4,696,231	4,394,110	$ 4,469,216
30-59 Days Past Due and Still Accruing, % of Total Loans	0.13%		0.12%
60-89 Days Past Due and Still Accruing, % of Total Loans	0.03%		0.04%
Greater Than 90 Days Past Due and Still Accruing, % of Total Loans	0.03%		0.01%
Total Past Due And Still Accruing, % of Total Loans	0.19%		0.17%
Accruing Current Balances, % of Total Loans	98.58%		98.57%
Acquired Impaired Loans, % of Total Loans	0.52%		0.75%
Nonaccrual Loans, % of Total Loans	0.71%		0.51%
% of Total Loans	100.00%		100.00%
Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	$ 1,494		$ 1,785
Accruing Current Balances	1,368,660		1,277,748
Acquired Impaired Loans	5,235	9,691	6,183
Nonaccrual Loans	12,705		2,015
Total Loans	1,388,094	1,266,717	1,287,731
Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	2,629		3,967
Accruing Current Balances	232,567		257,234
Acquired Impaired Loans	788	899	860
Nonaccrual Loans	4,733		4,967
Total Loans	240,717	268,711	267,028
Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	1,460		1,023
Accruing Current Balances	516,463		444,642
Acquired Impaired Loans	243	368	369
Nonaccrual Loans	2,110		3,995
Total Loans	520,276	438,158	450,029
30–59 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	6,325		5,490
30–59 Days Past Due and Still Accruing | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	673		1,507
30–59 Days Past Due and Still Accruing | Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	1,708		3,176
30–59 Days Past Due and Still Accruing | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	593		392
60–89 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	1,271		1,802
60–89 Days Past Due and Still Accruing | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	136		278
60–89 Days Past Due and Still Accruing | Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	364		638
60–89 Days Past Due and Still Accruing | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	771		346
Greater Than 90 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	1,338		438
Greater Than 90 Days Past Due and Still Accruing | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	685		0
Greater Than 90 Days Past Due and Still Accruing | Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	557		153
Greater Than 90 Days Past Due and Still Accruing | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	96		285
Owner-occupied commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	998		656
Accruing Current Balances	1,085,008		1,063,306
Acquired Impaired Loans	7,401	10,673	12,122
Nonaccrual Loans	3,346		2,078
Total Loans	1,096,753	1,057,645	1,078,162
Owner-occupied commercial | 30–59 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	998		116
Owner-occupied commercial | 60–89 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	0		540
Owner-occupied commercial | Greater Than 90 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	0		0
Commercial mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	1,320		167
Accruing Current Balances	1,136,982		1,143,180
Acquired Impaired Loans	9,969	12,880	10,386
Nonaccrual Loans	8,803		9,821
Total Loans	1,157,074	1,153,903	1,163,554
Commercial mortgages | 30–59 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	1,320		167
Commercial mortgages | 60–89 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	0		0
Commercial mortgages | Greater Than 90 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	0		0
Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	1,033		132
Accruing Current Balances	289,499		218,886
Acquired Impaired Loans	946	3,592	3,694
Nonaccrual Loans	1,839		0
Total Loans	293,317	$ 208,976	222,712
Construction | 30–59 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	1,033		132
Construction | 60–89 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	0		0
Construction | Greater Than 90 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	$ 0		$ 0