Allowance for Loan Losses and Credit Quality Information - Summary of Nonaccrual and Past Due Loans (Detail) - USD ($) $ in Thousands	6 Months Ended		12 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due and Still Accruing, Total	$ 15,728		$ 28,205
Accruing Current Balances, Total	3,790,134		3,692,582
Acquired Impaired Loans, Total	27,072	$ 15,017	32,687
Nonaccrual Loans, Total	14,581		21,165
Total Loans	$ 3,847,515	3,343,505	$ 3,774,639
30-59 Days Past Due and Still Accruing, % of Total Loans	0.22%		0.24%
60-89 Days Past Due and Still Accruing, % of Total Loans	0.17%		0.03%
Greater Than 90 Days Past Due and Still Accruing, % of Total Loans	0.02%		0.48%
Total Past Due And Still Accruing, % of Total Loans	0.41%		0.75%
Accruing Current Balances, % of Total Loans	98.51%		97.83%
Acquired Impaired Loans, % of Total Loans	0.70%		0.86%
Nonaccrual Loans, % of Total Loans	0.38%		0.56%
% of Total Loans	100.00%		100.00%
Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due and Still Accruing, Total	$ 937		$ 14,311
Accruing Current Balances, Total	1,103,908		1,028,973
Acquired Impaired Loans, Total	1,711	3,159	12,985
Nonaccrual Loans, Total	2,054		5,328
Total Loans	1,108,610	936,765	1,061,597
Residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due and Still Accruing, Total	5,547		6,135
Accruing Current Balances, Total	224,341		245,307
Acquired Impaired Loans, Total	946	460	950
Nonaccrual Loans, Total	5,823		7,287
Total Loans	236,657	212,797	259,679
Consumer Loan Commitments [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due and Still Accruing, Total	1,177		1,510
Accruing Current Balances, Total	369,209		354,599
Acquired Impaired Loans, Total	4	8	7
Nonaccrual Loans, Total	4,244		4,133
Total Loans	374,634	328,884	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	8,476		9,025
Financing Receivables, 30 to 59 Days Past Due [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due and Still Accruing, Total	720		1,686
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	4,598		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	734		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	6,532		1,148
Financing Receivables, 60 to 89 Days Past Due [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due and Still Accruing, Total	132		270
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	533		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	224		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	720		18,032
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due and Still Accruing, Total	85		12,355
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	416		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	219		252
Owner- Occupied Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due and Still Accruing, Total			5,816
Accruing Current Balances, Total	927,735		869,048
Acquired Impaired Loans, Total	9,757	2,027	4,688
Nonaccrual Loans, Total	838		1,091
Total Loans	938,330	793,062	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			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	8,067		433
Accruing Current Balances, Total	969,996		952,426
Acquired Impaired Loans, Total	10,560	5,884	10,513
Nonaccrual Loans, Total	1,622		3,326
Total Loans	990,245	870,152	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	2,424		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	5,643		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 [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Current Balances, Total	194,945		242,229
Acquired Impaired Loans, Total	4,094	3,479	3,544
Total Loans	$ 199,039	$ 201,845	$ 245,773