Allowance for Loan Losses - Age Analysis of Past Due Loans Segregated by Class of Loans (Detail) - USD ($) $ in Thousands		Sep. 30, 2015		Dec. 31, 2014
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current		$ 4,884,947		$ 3,850,597
Total nonaccrual loans		23,808		12,457
Total past due		54,810		21,119
Total Loans		4,939,757		3,871,716
Commercial Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current		2,092,139		1,674,930
Total nonaccrual loans		7,919		4,435
Total past due		19,446		7,306
Total Loans		2,111,585		1,682,236
Commercial Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current		374,001		214,174
Total nonaccrual loans		5,008		1,974
Total past due		10,327		1,974
Total Loans		384,328		216,148
Residential Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current		614,609		485,465
Total nonaccrual loans		3,242		2,336
Total past due		10,642		4,121
Total Loans		625,251		489,586
Home Equity [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current		463,145		414,303
Total nonaccrual loans		2,147		2,059
Total past due		4,553		4,260
Total Loans		467,698		418,563
Installment and Other Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current		90,624		65,111
Total nonaccrual loans		122		31
Total past due		498		456
Total Loans		91,122		65,567
Consumer Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current		8,064		2,508
Total Loans		8,064		2,508
Loans Held for Sale [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current		13,794		2,970
Total Loans		13,794		2,970
30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due		21,176		6,448
30 to 59 Days Past Due [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due		8,987		2,548
30 to 59 Days Past Due [Member] | Commercial Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due		3,955
30 to 59 Days Past Due [Member] | Residential Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due		3,563		565
30 to 59 Days Past Due [Member] | Home Equity [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due		2,144		1,756
30 to 59 Days Past Due [Member] | Installment and Other Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due		344		352
60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due		8,297		2,214
60 to 89 Days Past Due [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due		1,567		323
60 to 89 Days Past Due [Member] | Commercial Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due		1,364		0
60 to 89 Days Past Due [Member] | Residential Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due		3,281		1,220
60 to 89 Days Past Due [Member] | Home Equity [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due		262		445
60 to 89 Days Past Due [Member] | Installment and Other Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due		32		73
90 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due		1,529	[1]	0
90 Days Past Due [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	[1]	973
90 Days Past Due [Member] | Commercial Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	[1]	0
90 Days Past Due [Member] | Residential Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	[1]	556
90 Days Past Due [Member] | Home Equity [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	[1]	0
Commercial and Industrial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current		1,228,571		991,136
Total nonaccrual loans		5,370		1,622
Total past due		9,344		3,002
Total Loans		1,237,915		994,138
Commercial and Industrial [Member] | 30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due		2,183		1,227
Commercial and Industrial [Member] | 60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due		$ 1,791		$ 153

[1] Represents acquired loans that were recorded at fair value at the acquisition date.