Loans - Impaired Financing Receivables with Related and not Related Allowances (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Financing Receivable, Impaired [Line Items]
Average Recorded Investment, Total	$ 19,157	$ 22,757	$ 24,964
Interest Income Recognized, Total	329	17	20
Cash Basis Interest Recognized, Total	329	17	20
Commercial, Industrial, and Agricultural [Member]
Financing Receivable, Impaired [Line Items]
Average Recorded Investment, Related Allowance	4,621	1,989	3,083
Interest Income Recognized, Related Allowance	73	7	4
Cash Basis Interest Recognized, Related Allowance	73	7	4
Average Recorded Investment, No Related Allowance	1,972	2,124	3,217
Interest Income Recognized, No Related Allowance	31	0	0
Cash Basis Interest Recognized, No Related Allowance	31	0	0
Commercial Mortgages [Member]
Financing Receivable, Impaired [Line Items]
Average Recorded Investment, Related Allowance	6,374	6,572	5,504
Interest Income Recognized, Related Allowance	85	3	3
Cash Basis Interest Recognized, Related Allowance	85	3	3
Average Recorded Investment, No Related Allowance	5,868	11,885	12,723
Interest Income Recognized, No Related Allowance	78	0	0
Cash Basis Interest Recognized, No Related Allowance	78	0	0
Residential Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Average Recorded Investment, Related Allowance	240	101	334
Interest Income Recognized, Related Allowance	46	7	13
Cash Basis Interest Recognized, Related Allowance	46	7	13
Average Recorded Investment, No Related Allowance	82	86	103
Interest Income Recognized, No Related Allowance	16	0	0
Cash Basis Interest Recognized, No Related Allowance	$ 16	$ 0	$ 0

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Average amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 15

-Subparagraph (c)(1)

-URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5212-111524

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 17

-URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5258-111524

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Amount of interest income recognized that the financing receivables (with related allowance for credit losses and without a related allowance for credit losses) were impaired.

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+ References

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Amount of interest income, recognized on accrual basis, on financing receivables that are recognized as impaired, for which there is no related allowance for credit losses.

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Amount of interest income, recognized on accrual basis, on financing receivables, for which there is a related allowance for credit losses and are recognized as impaired.

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Amount of interest income on financing receivables, for which there is a related allowance for credit losses, recognized on a cash-basis method of accounting that the financing receivables were impaired.

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