Allowance for Loan Losses and Credit Quality Information - Summary of Nonaccrual and Past Due Loans (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	$ 8,592	$ 7,730
Accruing Current Balances	4,738,412	4,404,996
Acquired Impaired Loans	21,654	33,614	$ 32,687
Nonaccrual Loans	36,436	22,876
Total Loans	$ 4,805,094	$ 4,469,216	3,774,639
30-59 Days Past Due and Still Accruing, % of Total Loans	0.14%	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	0.01%	0.01%
Total Past Due And Still Accruing, % of Total Loans	0.18%	0.17%
Accruing Current Balances, % of Total Loans	98.61%	98.57%
Acquired Impaired Loans, % of Total Loans	0.45%	0.75%
Nonaccrual Loans, % of Total Loans	0.76%	0.51%
% of Total Loans	100.00%	100.00%
Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	$ 1,050	$ 1,785
Accruing Current Balances	1,440,291	1,277,748
Acquired Impaired Loans	4,156	6,183	12,985
Nonaccrual Loans	19,057	2,015
Total Loans	1,464,554	1,287,731	1,061,597
Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	3,145	3,967
Accruing Current Balances	225,434	257,234
Acquired Impaired Loans	784	860	950
Nonaccrual Loans	4,124	4,967
Total Loans	233,487	267,028	259,679
Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	1,685	1,023
Accruing Current Balances	554,634	444,642
Acquired Impaired Loans	247	369	7
Nonaccrual Loans	1,927	3,995
Total Loans	558,493	450,029	360,249
30–59 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	6,614	5,490
30–59 Days Past Due and Still Accruing | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	1,050	1,507
30–59 Days Past Due and Still Accruing | Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	2,058	3,176
30–59 Days Past Due and Still Accruing | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	1,117	392
60–89 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	1,517	1,802
60–89 Days Past Due and Still Accruing | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	0	278
60–89 Days Past Due and Still Accruing | Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	731	638
60–89 Days Past Due and Still Accruing | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	463	346
Greater Than 90 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	461	438
Greater Than 90 Days Past Due and Still Accruing | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	0	0
Greater Than 90 Days Past Due and Still Accruing | Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	356	153
Greater Than 90 Days Past Due and Still Accruing | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	105	285
Owner-occupied commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	2,302	656
Accruing Current Balances	1,067,488	1,063,306
Acquired Impaired Loans	5,803	12,122	4,688
Nonaccrual Loans	3,654	2,078
Total Loans	1,079,247	1,078,162	880,643
Owner-occupied commercial | 30–59 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	2,069	116
Owner-occupied commercial | 60–89 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	233	540
Owner-occupied commercial | Greater Than 90 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	0	0
Commercial mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	410	167
Accruing Current Balances	1,171,701	1,143,180
Acquired Impaired Loans	9,724	10,386	10,513
Nonaccrual Loans	5,870	9,821
Total Loans	1,187,705	1,163,554	966,698
Commercial mortgages | 30–59 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	320	167
Commercial mortgages | 60–89 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	90	0
Commercial mortgages | Greater Than 90 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	0	0
Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	0	132
Accruing Current Balances	278,864	218,886
Acquired Impaired Loans	940	3,694	3,544
Nonaccrual Loans	1,804	0
Total Loans	281,608	222,712	$ 245,773
Construction | 30–59 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	0	132
Construction | 60–89 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
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]
Past Due And Still Accruing	$ 0	$ 0