Allowance for Loan Losses - Age Analysis of Past Due Loans Segregated by Class of Loans (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	$ 4,653,941	$ 3,850,597
Loans, 30 to 59 Days Past Due	11,393	6,448
Loans, 60 to 89 Days Past Due	1,327	2,214
Loans, 90 Days Past Due	5,029	0
Total nonaccrual loans	18,134	12,457
Loans, Past Due, Total	35,883	21,119
Total Loans	4,689,824us-gaap_LoansReceivableNet	3,871,716us-gaap_LoansReceivableNet
Commercial Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	2,138,815	1,674,930
Loans, 30 to 59 Days Past Due	2,955	2,548
Loans, 60 to 89 Days Past Due	522	323
Loans, 90 Days Past Due	1,735
Total nonaccrual loans	8,386	4,435
Loans, Past Due, Total	13,598	7,306
Total Loans	2,152,413	1,682,236
Commercial and Industrial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	1,202,897	991,136
Loans, 30 to 59 Days Past Due	4,089	1,227
Loans, 60 to 89 Days Past Due	517	153
Loans, 90 Days Past Due	0	0
Total nonaccrual loans	3,550	1,622
Loans, Past Due, Total	8,156	3,002
Total Loans	1,211,053	994,138
Commercial Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	281,084	214,174
Loans, 30 to 59 Days Past Due	969
Loans, 90 Days Past Due	2,140
Total nonaccrual loans	1,973	1,974
Loans, Past Due, Total	5,082	1,974
Total Loans	286,166	216,148
Residential Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	516,515	485,465
Loans, 30 to 59 Days Past Due	1,548	565
Loans, 60 to 89 Days Past Due	63	1,220
Loans, 90 Days Past Due	1,154
Total nonaccrual loans	2,226	2,336
Loans, Past Due, Total	4,991	4,121
Total Loans	521,506	489,586
Home Equity [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	438,629	414,303
Loans, 30 to 59 Days Past Due	1,583	1,756
Loans, 60 to 89 Days Past Due	198	445
Total nonaccrual loans	1,986	2,059
Loans, Past Due, Total	3,767	4,260
Total Loans	442,396	418,563
Installment and Other Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	65,465	65,111
Loans, 30 to 59 Days Past Due	249	352
Loans, 60 to 89 Days Past Due	27	73
Total nonaccrual loans	13	31
Loans, Past Due, Total	289	456
Total Loans	65,754	65,567
Consumer Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	4,410	2,508
Total Loans	4,410	2,508
Loans Held for Sale [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, Current	6,126	2,970
Total Loans	$ 6,126	$ 2,970

Financing receivables that are less than 60 days past due but more than 29 days past due.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 7A

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953401-111524

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Financing receivables that are current.

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 7

-Subparagraph (d)

-URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5111-111524

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Amount after allowance and deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes loans held for sale. Excludes loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 7

-Subparagraph a

-Article 9

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.7(a))

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