Allowance for Loan Losses - Age Analysis of Past Due Loans Segregated by Class of Loans (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	$ 4,758,474	$ 3,850,597
Total nonaccrual loans	19,139	12,457
Total past due	53,658	21,119
Total Loans	4,812,132	3,871,716
Commercial Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	2,032,731	1,674,930
Total nonaccrual loans	7,309	4,435
Total past due	22,204	7,306
Total Loans	2,054,935	1,682,236
Commercial and Industrial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	1,231,221	991,136
Total nonaccrual loans	3,143	1,622
Total past due	8,161	3,002
Total Loans	1,239,382	994,138
Construction Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	333,897	214,174
Total nonaccrual loans	4,472	1,974
Total past due	11,086	1,974
Total Loans	344,983	216,148
Residential Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	593,043	485,465
Total nonaccrual loans	2,010	2,336
Total past due	6,459	4,121
Total Loans	599,502	489,586
Home Equity [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	453,279	414,303
Total nonaccrual loans	2,026	2,059
Total past due	4,534	4,260
Total Loans	457,813	418,563
Consumer Borrower [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	93,223	65,111
Total nonaccrual loans	179	31
Total past due	1,214	456
Total Loans	94,437	65,567
Consumer Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	7,446	2,508
Total Loans	7,446	2,508
Loans Held for Sale [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	13,634	2,970
Total Loans	13,634	2,970
30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	20,969	6,448
30 to 59 Days Past Due [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	9,656	2,548
30 to 59 Days Past Due [Member] | Commercial and Industrial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	2,875	1,227
30 to 59 Days Past Due [Member] | Construction Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	4,474
30 to 59 Days Past Due [Member] | Residential Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	1,732	565
30 to 59 Days Past Due [Member] | Home Equity [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	1,971	1,756
30 to 59 Days Past Due [Member] | Consumer Borrower [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	261	352
60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	8,635	2,214
60 to 89 Days Past Due [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	3,518	323
60 to 89 Days Past Due [Member] | Commercial and Industrial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	2,143	153
60 to 89 Days Past Due [Member] | Construction Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due		0
60 to 89 Days Past Due [Member] | Residential Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	1,770	1,220
60 to 89 Days Past Due [Member] | Home Equity [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	430	445
60 to 89 Days Past Due [Member] | Consumer Borrower [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	774	73
90 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	4,915	$ 0
90 Days Past Due [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	1,721
90 Days Past Due [Member] | Construction Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	2,140
90 Days Past Due [Member] | Residential Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	947
90 Days Past Due [Member] | Home Equity [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	$ 107