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, 2016	Mar. 31, 2015	Dec. 31, 2015
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due and Still Accruing, Total	$ 9,604		$ 28,205
Accruing Current Balances, Total	3,742,034		3,692,582
Acquired Impaired Loans, Total	32,284	$ 15,336	32,687
Nonaccrual Loans, Total	19,791		21,165
Total Loans	$ 3,803,713	3,234,959	$ 3,774,639
30-59 Days Past Due and Still Accruing, % of Total Loans	0.19%		0.24%
60-89 Days Past Due and Still Accruing, % of Total Loans	0.06%		0.03%
Greater Than 90 Days Past Due and Still Accruing, % of Total Loans			0.48%
Total Past Due And Still Accruing, % of Total Loans	0.25%		0.75%
Accruing Current Balances, % of Total Loans	98.38%		97.83%
Acquired Impaired Loans, % of Total Loans	0.85%		0.86%
Nonaccrual Loans, % of Total Loans	0.52%		0.56%
% of Total Loans	100.00%		100.00%
Residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due and Still Accruing, Total	$ 4,750		$ 6,135
Accruing Current Balances, Total	238,482		245,307
Acquired Impaired Loans, Total	955	496	950
Nonaccrual Loans, Total	6,878		7,287
Total Loans	251,065	216,197	259,679
Consumer Loan Commitments [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due and Still Accruing, Total	1,317		1,510
Accruing Current Balances, Total	354,219		354,599
Acquired Impaired Loans, Total	5	8	7
Nonaccrual Loans, Total	4,115		4,133
Total Loans	359,656	324,232	360,249
Financing Receivables, 30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due and Still Accruing, Total	7,366		9,025
Financing Receivables, 30 to 59 Days Past Due [Member] | Residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due and Still Accruing, Total	4,332		5,263
Financing Receivables, 30 to 59 Days Past Due [Member] | Consumer Loan Commitments [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due and Still Accruing, Total	851		1,222
Financing Receivables, 60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due and Still Accruing, Total	2,111		1,148
Financing Receivables, 60 to 89 Days Past Due [Member] | Residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due and Still Accruing, Total	418		621
Financing Receivables, 60 to 89 Days Past Due [Member] | Consumer Loan Commitments [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due and Still Accruing, Total	339		36
Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due and Still Accruing, Total	127		18,032
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due and Still Accruing, Total			251
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Consumer Loan Commitments [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due and Still Accruing, Total	127		252
Commercial Loan Commitments [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due and Still Accruing, Total	2,732		14,311
Accruing Current Balances, Total	1,062,856		1,028,973
Acquired Impaired Loans, Total	12,600	3,192	12,985
Nonaccrual Loans, Total	4,933		5,328
Total Loans	1,083,121	923,070	1,061,597
Commercial Loan Commitments [Member] | Financing Receivables, 30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due and Still Accruing, Total	1,378		1,686
Commercial Loan Commitments [Member] | Financing Receivables, 60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due and Still Accruing, Total	1,354		270
Commercial Loan Commitments [Member] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due and Still Accruing, Total			12,355
Owner- Occupied Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due and Still Accruing, Total	678		5,816
Accruing Current Balances, Total	888,907		869,048
Acquired Impaired Loans, Total	4,603	2,133	4,688
Nonaccrual Loans, Total	1,269		1,091
Total Loans	895,457	797,400	880,643
Owner- Occupied Commercial [Member] | Financing Receivables, 30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due and Still Accruing, Total	678		713
Owner- Occupied Commercial [Member] | Financing Receivables, 60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due and Still Accruing, Total			217
Owner- Occupied Commercial [Member] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due and Still Accruing, Total			4,886
Commercial Mortgages [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due and Still Accruing, Total	127		433
Accruing Current Balances, Total	973,786		952,426
Acquired Impaired Loans, Total	10,557	5,877	10,513
Nonaccrual Loans, Total	2,596		3,326
Total Loans	987,066	820,836	966,698
Commercial Mortgages [Member] | Financing Receivables, 30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due and Still Accruing, Total	127		141
Commercial Mortgages [Member] | Financing Receivables, 60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due and Still Accruing, Total			4
Commercial Mortgages [Member] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due and Still Accruing, Total			288
Construction Loan Commitments [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Current Balances, Total	223,784		242,229
Acquired Impaired Loans, Total	3,564	3,630	3,544
Total Loans	$ 227,348	$ 153,224	$ 245,773