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Loans and Allowance for Loan Losses - Impaired Loans Segregated by Class of Loans (Detail) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Unpaid Principal Balance | $ 49,391 | $ 47,665
Recorded Investment with Allowance | 39,688 | 35,709
Related Allowance | 5,647 | 5,797
Average Recorded Investment | 35,181 | 35,105
Real Estate [Member] | Non-residential Real Estate Owner Occupied [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Unpaid Principal Balance | 2,011 | 894
Recorded Investment with Allowance | 1,945 | 806
Related Allowance | 141 | 101
Average Recorded Investment | 1,858 | 825
Real Estate [Member] | Non-residential Real Estate Other [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Unpaid Principal Balance | 10,323 | 7,742
Recorded Investment with Allowance | 10,240 | 5,688
Related Allowance | 496 | 574
Average Recorded Investment | 3,975 | 5,854
Real Estate [Member] | Residential Real Estate Permanent Mortgage [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Unpaid Principal Balance | 1,745 | 1,878
Recorded Investment with Allowance | 1,542 | 1,683
Related Allowance | 146 | 124
Average Recorded Investment | 1,440 | 1,612
Real Estate [Member] | Residential Real Estate All Other [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Unpaid Principal Balance | 5,837 | 5,871
Recorded Investment with Allowance | 5,549 | 5,614
Related Allowance | 2,135 | 1,538
Average Recorded Investment | 5,258 | 5,445
Commercial and Financial [Member] | Non-consumer Non-real Estate [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Unpaid Principal Balance | 18,101 | 12,015
Recorded Investment with Allowance | 11,158 | 6,272
Related Allowance | 2,412 | 1,457
Average Recorded Investment | 11,131 | 6,478
Consumer Non-real Estate [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Unpaid Principal Balance | 545 | 686
Recorded Investment with Allowance | 514 | 650
Related Allowance | 127 | 133
Average Recorded Investment | 541 | 788
Other Loans [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Unpaid Principal Balance | 6,092 | 9,799
Recorded Investment with Allowance | 5,595 | 8,415
Related Allowance | 178 | 1,870
Average Recorded Investment | 7,439 | 8,062
Acquired Loans [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Unpaid Principal Balance | 4,737 | 8,780
Recorded Investment with Allowance | 3,145 | 6,581
Related Allowance | 12 |
Average Recorded Investment | $ 3,539 | $ 6,041

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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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