Allowance for Loan Losses and Credit Quality Information - Schedule of Allowance for Loan Losses and Loan Balances (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2013	Mar. 31, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	$ 43,922	$ 53,080
Charge-offs	(3,707)	(6,082)
Recoveries	502	555
Provision	2,231	8,245
Ending balance	42,948	55,798
Loans individually evaluated for impairment	3,849	8,677
Loans collectively evaluated for impairment	39,099	47,121
Ending balance	42,948	55,798
Loans individually evaluated for impairment	56,498	82,792
Loans collectively evaluated for impairment	2,730,939	2,696,869
Ending balance	2,787,437	2,779,661
Commercial Loan Commitments [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	13,663	15,067
Charge-offs	(256)	(2,331)
Recoveries	226	53
Provision	(865)	(1,164)
Ending balance	12,768	11,625
Loans individually evaluated for impairment	868	1,615
Loans collectively evaluated for impairment	11,900	10,010
Ending balance	12,768	11,625
Loans individually evaluated for impairment	4,991	6,526
Loans collectively evaluated for impairment	727,831	792,293
Ending balance	732,822	798,819
Owner-Occupied Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	6,108	9,235
Charge-offs	(1)	(502)
Recoveries	12	6
Provision	219	(1,734)
Ending balance	6,338	7,005
Loans individually evaluated for impairment	47	2,191
Loans collectively evaluated for impairment	6,291	4,814
Ending balance	6,338	7,005
Loans individually evaluated for impairment	13,263	21,760
Loans collectively evaluated for impairment	752,961	665,180
Ending balance	766,224	686,940
Commercial Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	8,079	7,556
Charge-offs	(1,697)	(190)
Recoveries	3	313
Provision	808	2,851
Ending balance	7,193	10,530
Loans individually evaluated for impairment	2,000	1,156
Loans collectively evaluated for impairment	5,193	9,374
Ending balance	7,193	10,530
Loans individually evaluated for impairment	11,240	11,625
Loans collectively evaluated for impairment	622,510	605,601
Ending balance	633,750	617,226
Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	6,456	4,074
Charge-offs	(19)	(1,506)
Recoveries	15	28
Provision	333	6,321
Ending balance	6,785	8,917
Loans individually evaluated for impairment		2,750
Loans collectively evaluated for impairment	6,785	6,167
Ending balance	6,785	8,917
Loans individually evaluated for impairment	1,216	24,246
Loans collectively evaluated for impairment	132,265	99,937
Ending balance	133,481	124,183
Residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	3,124	6,544
Charge-offs	(440)	(324)
Recoveries	18	25
Provision	579	155
Ending balance	3,281	6,400
Loans individually evaluated for impairment	922	869
Loans collectively evaluated for impairment	2,359	5,531
Ending balance	3,281	6,400
Loans individually evaluated for impairment	19,578	15,723
Loans collectively evaluated for impairment	218,276	251,310
Ending balance	237,854	267,033
Consumer Loan Commitments [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	5,631	10,604
Charge-offs	(1,294)	(1,229)
Recoveries	228	130
Provision	1,176	748
Ending balance	5,741	10,253
Loans individually evaluated for impairment	12	96
Loans collectively evaluated for impairment	5,729	10,157
Ending balance	5,741	10,253
Loans individually evaluated for impairment	6,210	2,912
Loans collectively evaluated for impairment	277,096	282,548
Ending balance	283,306	285,460
Complexity Risk [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	861
Charge-offs
Recoveries
Provision	(19)	1,068
Ending balance	842	1,068
Loans individually evaluated for impairment
Loans collectively evaluated for impairment	842	1,068
Ending balance	842	1,068
Loans individually evaluated for impairment
Loans collectively evaluated for impairment
Ending balance

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Reflects the gross amount charged against earnings during the period as credit losses (bad debt expense) based on review of the loan and lease portfolio and application of the methodology for estimating credit losses.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 114

-Paragraph 20

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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(d))

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Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 15

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=7512638&loc=d3e5212-111524

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 7

-Subparagraph d

-Article 9

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- Definition

Reflects a reduction in bad debt expense attributable to collections of loans and lease receivables which had previously been fully or partially charged off as bad debts.

+ References

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- Definition

Reflects the amount of loans and leases that have been removed, or charged off, from both loan receivables and the reserve for credit losses, typically because they are considered to be not salvageable or have been purchased by unconsolidated collection entities.

+ References

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- Definition

A valuation allowance for financing receivables that are expected to be uncollectible.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 11B

-Subparagraph (c)(1)

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- Definition

The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 11B

-Subparagraph (g)

-URI http://asc.fasb.org/extlink&oid=7512638&loc=SL6953423-111524

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- Definition

Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 11B

-Subparagraph (g)

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- Definition

The balance of financing receivables that were collectively evaluated for impairment.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

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-Subparagraph (h)

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- Definition

The balance of financing receivables that were individually evaluated for impairment.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

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- Definition

Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 5

-Paragraph 8, 9

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 30

-Section 50

-Paragraph 2

-Subparagraph (b)(1)(ii)

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.7(d))

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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-Publisher OTS

-Name Federal Regulation (FR)

-Number Title 12

-Section 563c.102

-Paragraph 8

-Subparagraph a(ii)

-Chapter V

-Subsection I

-LegacyDoc This is a non-GAAP reference that was included in the 2009 taxonomy. It will be removed from future versions of this taxonomy.

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 7

-Subparagraph d

-Article 9

Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 114

-Paragraph 20

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 7: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 6

-Section L

-Subsection 1

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- Definition

Amount representing an agreement for an unconditional promise by the maker to pay the entity (holder) a definite sum of money at a future date. Such amount may include accrued interest receivable in accordance with the terms of the note. The note also may contain provisions and related items including a discount or premium, payable on demand, secured, or unsecured, interest bearing or noninterest bearing, among a myriad of other features and characteristics. Excludes amounts related to receivables held-for-sale.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.10(3))

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 10

-Subparagraph 3

-Article 9

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