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, 2015	Dec. 31, 2014	Jun. 30, 2014
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due and Still Accruing, Total	$ 5,710	$ 7,381
Accruing Current Balances, Total	3,295,059	3,155,173
Acquired Impaired Loans ,Total	15,017	15,893
Nonaccrual Loans, Total	27,719	24,051
Total Loans	$ 3,343,505	$ 3,202,498	$ 3,041,477
30-59 Days Past Due and Still Accruing, % of Total Loans	0.13%	0.20%
60-89 Days Past Due and Still Accruing, % of Total Loans	0.03%	0.03%
Greater Than 90 Days Past Due and Still Accruing, % of Total Loans	0.01%	0.00%
Total Past Due And Still Accruing, % of Total Loans	0.17%	0.23%
Accruing Current Balances, % of Total Loans	98.55%	98.52%
Acquired Impaired Loans, % of Total Loans	0.45%	0.50%
Nonaccrual Loans, % of Total Loans	0.83%	0.75%
% of Total Loans	100.00%	100.00%
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	$ 4,425	$ 6,431
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	1,132	946
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	153	4
Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due and Still Accruing, Total	135	715
Accruing Current Balances, Total	923,924	913,382
Acquired Impaired Loans ,Total	3,159	3,269
Nonaccrual Loans, Total	9,547	2,706
Total Loans	936,765	920,072	857,354
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	135	715
Owner Occupied Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due and Still Accruing, Total	144	393
Accruing Current Balances, Total	789,501	783,466
Acquired Impaired Loans ,Total	2,027	2,264
Nonaccrual Loans, Total	1,390	2,475
Total Loans	793,062	788,598	778,257
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	144	393
Commercial Mortgages [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due and Still Accruing, Total	476	203
Accruing Current Balances, Total	856,549	791,035
Acquired Impaired Loans ,Total	5,884	5,976
Nonaccrual Loans, Total	7,243	8,245
Total Loans	870,152	805,459	763,090
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	390	203
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	86
Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Current Balances, Total	198,366	138,634
Acquired Impaired Loans ,Total	3,479	3,863
Total Loans	201,845	142,497	119,333
Residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due and Still Accruing, Total	3,792	4,483
Accruing Current Balances, Total	201,878	206,266
Acquired Impaired Loans ,Total	460	512
Nonaccrual Loans, Total	6,667	7,068
Total Loans	212,797	218,329	210,923
Residential [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	2,706	3,879
Residential [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	933	604
Residential [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	153
Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due and Still Accruing, Total	1,163	1,587
Accruing Current Balances, Total	324,841	322,390
Acquired Impaired Loans ,Total	8	9
Nonaccrual Loans, Total	2,872	3,557
Total Loans	328,884	327,543	$ 312,520
Consumer [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	1,050	1,241
Consumer [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	$ 113	342
Consumer [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