Allowance for Loan Losses and Credit Quality Information - Summary of Nonaccrual and Past Due Loans (Detail) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	$ 7,469		$ 7,730
Accruing Current Balances	4,499,234		4,404,996
Acquired Impaired Loans	31,247	$ 32,284	33,614
Nonaccrual Loans	39,678		22,876
Total Loans	$ 4,577,628	3,803,713	$ 4,469,216
30-59 Days Past Due and Still Accruing, % of Total Loans	0.08%		0.12%
60-89 Days Past Due and Still Accruing, % of Total Loans	0.04%		0.04%
Greater Than 90 Days Past Due and Still Accruing, % of Total Loans	0.04%		0.01%
Total Past Due And Still Accruing, % of Total Loans	0.16%		0.17%
Accruing Current Balances, % of Total Loans	98.29%		98.56%
Acquired Impaired Loans, % of Total Loans	0.68%		0.75%
Nonaccrual Loans, % of Total Loans	0.87%		0.51%
% of Total Loans	100.00%		100.00%
Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	$ 1,250		$ 1,785
Accruing Current Balances	1,316,841		1,277,748
Acquired Impaired Loans	5,256	12,600	6,183
Nonaccrual Loans	16,520		2,015
Total Loans	1,339,867	1,083,121	1,287,731
Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	2,989		3,967
Accruing Current Balances	250,443		257,234
Acquired Impaired Loans	866	955	860
Nonaccrual Loans	4,957		4,967
Total Loans	259,255	251,065	267,028
Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	1,092		1,023
Accruing Current Balances	459,057		444,642
Acquired Impaired Loans	263	5	369
Nonaccrual Loans	4,486		3,995
Total Loans	464,898	359,656	450,029
30–59 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	4,006		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	783		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	2,020		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	374		392
60–89 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	1,698		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	467		278
60–89 Days Past Due and Still Accruing | Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	119		638
60–89 Days Past Due and Still Accruing | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	612		346
Greater Than 90 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	1,765		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	0		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	850		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	106		285
Owner-occupied commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	841		656
Accruing Current Balances	1,075,391		1,063,306
Acquired Impaired Loans	12,040	4,603	12,122
Nonaccrual Loans	4,020		2,078
Total Loans	1,092,292	895,457	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	341		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	500		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	794		167
Accruing Current Balances	1,144,617		1,143,180
Acquired Impaired Loans	10,450	10,557	10,386
Nonaccrual Loans	9,695		9,821
Total Loans	1,165,556	987,066	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	451		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	343		0
Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	503		132
Accruing Current Balances	252,885		218,886
Acquired Impaired Loans	2,372	3,564	3,694
Nonaccrual Loans	0		0
Total Loans	255,760	$ 227,348	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	37		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	$ 466		$ 0