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Loans and Allowance for Loan Losses - Impaired Loans Segregated by Class of Loans (Detail) - USD ($) $ in Thousands | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2018 | Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Unpaid Principal Balance | $ 56,964 | $ 49,391
Recorded Investment with Allowance | 45,150 | 39,688
Related Allowance | 5,530 | 5,647
Average Recorded Investment | 47,054 | 35,181
Real Estate [Member] | Non-residential Real Estate Owner Occupied [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Unpaid Principal Balance | 8,187 | 2,011
Recorded Investment with Allowance | 8,063 | 1,945
Related Allowance | 353 | 141
Average Recorded Investment | 7,877 | 1,858
Real Estate [Member] | Non-residential Real Estate Other [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Unpaid Principal Balance | 8,262 | 10,323
Recorded Investment with Allowance | 7,753 | 10,240
Related Allowance | 333 | 496
Average Recorded Investment | 8,678 | 3,975
Real Estate [Member] | Residential Real Estate Permanent Mortgage [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Unpaid Principal Balance | 1,842 | 1,745
Recorded Investment with Allowance | 1,634 | 1,542
Related Allowance | 145 | 146
Average Recorded Investment | 1,746 | 1,440
Real Estate [Member] | Residential Real Estate All Other [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Unpaid Principal Balance | 6,762 | 5,837
Recorded Investment with Allowance | 6,507 | 5,549
Related Allowance | 2,310 | 2,135
Average Recorded Investment | 6,678 | 5,258
Commercial and Financial [Member] | Non-consumer Non-real Estate [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Unpaid Principal Balance | 19,458 | 18,101
Recorded Investment with Allowance | 12,065 | 11,158
Related Allowance | 2,234 | 2,412
Average Recorded Investment | 13,359 | 11,131
Consumer Non-real Estate [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Unpaid Principal Balance | 897 | 545
Recorded Investment with Allowance | 825 | 514
Related Allowance | 108 | 127
Average Recorded Investment | 818 | 541
Other Loans [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Unpaid Principal Balance | 772 | 6,092
Recorded Investment with Allowance | 558 | 5,595
Related Allowance | 43 | 178
Average Recorded Investment | 456 | 7,439
Acquired Loans [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Unpaid Principal Balance | 10,784 | 4,737
Recorded Investment with Allowance | 7,745 | 3,145
Related Allowance | 4 | 12
Average Recorded Investment | $ 7,442 | $ 3,539

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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://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 + Details
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