Loans, Leases and Other Real Estate - Impaired Loans with and without Specific Allowances (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Financing Receivable, Impaired [Line Items]
Recorded Investment in Impaired Loans	$ 18,380	$ 22,591
Contractual Unpaid Principal Balance	19,791	23,461
Related Allowance	349	505
Average Investment in Impaired Loans	20,200	23,236
Interest Income Recognized	450	705
Commercial, secured by real estate
Financing Receivable, Impaired [Line Items]
Loans without related allowance, Recorded Investment in Impaired Loans	7,918	12,155
Loans with related allowance, Recorded Investment in Impaired Loans	6,982	5,381
Recorded Investment in Impaired Loans	14,900	17,536
Loans without related allowance, Contractual Unpaid Principal Balance	8,160	12,497
Loans with related allowance, Contractual Unpaid Principal Balance	7,309	5,721
Contractual Unpaid Principal Balance	15,469	18,218
Related Allowance	322	454
Loans without related allowance, Average Investment in Impaired Loans	7,147	12,774
Loans with related allowance, Average Investment in Impaired Loans	7,804	5,029
Average Investment in Impaired Loans	14,951	17,803
Loans without related allowance, Interest Income Recognized	136	366
Loans with related allowance, Interest Income Recognized	247	206
Interest Income Recognized	383	572
Commercial, industrial and other
Financing Receivable, Impaired [Line Items]
Loans without related allowance, Recorded Investment in Impaired Loans	1,329	618
Loans with related allowance, Recorded Investment in Impaired Loans	219	164
Recorded Investment in Impaired Loans	1,548	782
Loans without related allowance, Contractual Unpaid Principal Balance	1,610	618
Loans with related allowance, Contractual Unpaid Principal Balance	219	164
Contractual Unpaid Principal Balance	1,829	782
Related Allowance	8	9
Loans without related allowance, Average Investment in Impaired Loans	1,799	618
Loans with related allowance, Average Investment in Impaired Loans	221	283
Average Investment in Impaired Loans	2,020	901
Loans without related allowance, Interest Income Recognized	14	25
Loans with related allowance, Interest Income Recognized	9	14
Interest Income Recognized	23	39
Leases
Financing Receivable, Impaired [Line Items]
Loans without related allowance, Recorded Investment in Impaired Loans	301	0
Loans with related allowance, Recorded Investment in Impaired Loans	17	65
Recorded Investment in Impaired Loans	318	65
Loans without related allowance, Contractual Unpaid Principal Balance	597	0
Loans with related allowance, Contractual Unpaid Principal Balance	17	65
Contractual Unpaid Principal Balance	614	65
Related Allowance	8	30
Loans without related allowance, Average Investment in Impaired Loans	258	0
Loans with related allowance, Average Investment in Impaired Loans	17	29
Average Investment in Impaired Loans	275	29
Loans without related allowance, Interest Income Recognized	0	0
Loans with related allowance, Interest Income Recognized	0	0
Interest Income Recognized	0	0
Real estate - residential mortgage
Financing Receivable, Impaired [Line Items]
Loans without related allowance, Recorded Investment in Impaired Loans	0	963
Loans with related allowance, Recorded Investment in Impaired Loans	743	781
Recorded Investment in Impaired Loans	743	1,744
Loans without related allowance, Contractual Unpaid Principal Balance	0	980
Loans with related allowance, Contractual Unpaid Principal Balance	893	919
Contractual Unpaid Principal Balance	893	1,899
Related Allowance	4	4
Loans without related allowance, Average Investment in Impaired Loans	323	996
Loans with related allowance, Average Investment in Impaired Loans	754	940
Average Investment in Impaired Loans	1,077	1,936
Loans without related allowance, Interest Income Recognized	4	15
Loans with related allowance, Interest Income Recognized	15	27
Interest Income Recognized	19	42
Real estate - construction
Financing Receivable, Impaired [Line Items]
Loans without related allowance, Recorded Investment in Impaired Loans	0	1,471
Loans with related allowance, Recorded Investment in Impaired Loans	0	0
Recorded Investment in Impaired Loans	0	1,471
Loans without related allowance, Contractual Unpaid Principal Balance	0	1,471
Loans with related allowance, Contractual Unpaid Principal Balance	0	0
Contractual Unpaid Principal Balance	0	1,471
Related Allowance	0	0
Loans without related allowance, Average Investment in Impaired Loans	970	1,471
Loans with related allowance, Average Investment in Impaired Loans	0	0
Average Investment in Impaired Loans	970	1,471
Loans without related allowance, Interest Income Recognized	0	0
Loans with related allowance, Interest Income Recognized	0	0
Interest Income Recognized	0	0
Home equity and consumer
Financing Receivable, Impaired [Line Items]
Loans without related allowance, Recorded Investment in Impaired Loans	0	0
Loans with related allowance, Recorded Investment in Impaired Loans	871	993
Recorded Investment in Impaired Loans	871	993
Loans without related allowance, Contractual Unpaid Principal Balance	0	0
Loans with related allowance, Contractual Unpaid Principal Balance	986	1,026
Contractual Unpaid Principal Balance	986	1,026
Related Allowance	7	8
Loans without related allowance, Average Investment in Impaired Loans	0	6
Loans with related allowance, Average Investment in Impaired Loans	907	1,090
Average Investment in Impaired Loans	907	1,096
Loans without related allowance, Interest Income Recognized	0	0
Loans with related allowance, Interest Income Recognized	25	52
Interest Income Recognized	$ 25	$ 52

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

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=84173941&loc=SL6953423-111524

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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 15
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=84173941&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=84173941&loc=d3e5258-111524

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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 14A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953550-111524

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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 17
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5258-111524

Reference 2: 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=84173941&loc=d3e5212-111524

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