Allowance for Loan Losses and Credit Quality Information - Summary of Nonaccrual and Past Due Loans (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	$ 13,343	$ 8,592
Accruing Current Balances	4,835,916	4,738,412
Acquired Impaired Loans	14,944	21,654	$ 33,614
Nonaccrual Loans	30,054	36,436
Total Loans	$ 4,894,257	$ 4,805,094	4,469,216
30-59 Days Past Due and Still Accruing, % of Total Loans	0.17%	0.14%
60-89 Days Past Due and Still Accruing, % of Total Loans	0.08%	0.03%
Greater Than 90 Days Past Due and Still Accruing, % of Total Loans	0.02%	0.01%
Total Past Due And Still Accruing, % of Total Loans	0.27%	0.18%
Accruing Current Balances, % of Total Loans	98.81%	98.61%
Acquired Impaired Loans, % of Total Loans	0.31%	0.45%
Nonaccrual Loans, % of Total Loans	0.61%	0.76%
% of Total Loans	100.00%	100.00%
30–59 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	$ 8,426	$ 6,614
60–89 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	4,082	1,517
Greater Than 90 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	835	461
Owner-occupied commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	1,598	2,302
Accruing Current Balances	1,049,722	1,067,488
Acquired Impaired Loans	4,248	5,803	12,122
Nonaccrual Loans	4,406	3,654
Total Loans	1,059,974	1,079,247	1,078,162
Owner-occupied commercial | 30–59 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	733	2,069
Owner-occupied commercial | 60–89 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	865	233
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	2,296	410
Accruing Current Balances	1,148,988	1,171,701
Acquired Impaired Loans	7,504	9,724	10,386
Nonaccrual Loans	3,951	5,870
Total Loans	1,162,739	1,187,705	1,163,554
Commercial mortgages | 30–59 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	1,388	320
Commercial mortgages | 60–89 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	908	90
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	157	0
Accruing Current Balances	312,879	278,864
Acquired Impaired Loans	749	940	3,694
Nonaccrual Loans	2,781	1,804
Total Loans	316,566	281,608	222,712
Construction | 30–59 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	157	0
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
Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	4,717	1,050
Accruing Current Balances	1,452,185	1,440,291
Acquired Impaired Loans	1,531	4,156	6,183
Nonaccrual Loans	14,056	19,057
Total Loans	1,472,489	1,464,554	1,287,731
Commercial | 30–59 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	3,653	1,050
Commercial | 60–89 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	993	0
Commercial | Greater Than 90 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	71	0
Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	2,975	3,145
Accruing Current Balances	194,960	225,434
Acquired Impaired Loans	761	784	860
Nonaccrual Loans	2,854	4,124
Total Loans	201,550	233,487	267,028
Residential | 30–59 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	1,970	2,058
Residential | 60–89 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	345	731
Residential | Greater Than 90 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	660	356
Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	1,600	1,685
Accruing Current Balances	677,182	554,634
Acquired Impaired Loans	151	247	369
Nonaccrual Loans	2,006	1,927
Total Loans	680,939	558,493	$ 450,029
Consumer | 30–59 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	525	1,117
Consumer | 60–89 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	971	463
Consumer | Greater Than 90 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	$ 104	$ 105