Loans - Impaired Financing Receivables with Related Allowances (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
With no related allowance recorded:
Average Recorded Investment, Total	$ 23,224	$ 18,037	$ 21,414	$ 18,327
Interest Income Recognized, Total	137	153	391	522
Cash Basis Interest Recognized, Total	137	153	391	522
Commercial, Industrial, and Agricultural
With an allowance recorded:
Average Recorded Investment	3,460	1,190	2,672	1,413
Interest Income Recognized	11	20	54	56
Cash Basis Interest Recognized	11	20	54	56
With no related allowance recorded:
Average Recorded Investment	5,569	2,142	5,084	1,927
Interest Income Recognized	69	23	160	73
Cash Basis Interest Recognized	69	23	160	73
Commercial mortgages
With an allowance recorded:
Average Recorded Investment	9,042	9,724	9,147	12,497
Interest Income Recognized	37	77	111	293
Cash Basis Interest Recognized	37	77	111	293
With no related allowance recorded:
Average Recorded Investment	5,153	4,981	4,511	2,490
Interest Income Recognized	20	33	66	100
Cash Basis Interest Recognized	20	33	66	100
Residential Real Estate
With an allowance recorded:
Average Recorded Investment	0	0	0	0
Interest Income Recognized	0	0	0	0
Cash Basis Interest Recognized	0	0	0	0
With no related allowance recorded:
Average Recorded Investment	0	0	0	0
Interest Income Recognized	0	0	0	0
Cash Basis Interest Recognized	$ 0	$ 0	$ 0	$ 0

Impaired Financing Receivable, Average Recorded Investment With An Allowance Recorded [Abstract]

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- Definition

Impaired Financing Receivable, Average Recorded Investment With No Allowance Recorded [Abstract]

+ References

No definition available.

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- Definition

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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5212-111524

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5258-111524

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- Definition

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.

+ References

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- Definition

Amount of interest income recognized on a cash-basis method of accounting that the financing receivables (with related allowance for credit losses and without a related allowance for credit losses) were impaired.

+ References

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Amount of average investment of impaired financing receivables for which there is no related allowance for credit losses.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5258-111524

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5212-111524

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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 cash basis, on financing receivables, for which there is no related allowance for credit losses and are recognized as impaired.

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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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