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Loans and Allowance for Loan Losses - Impaired Loans Segregated by Class of Loans (Detail) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2018 | Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Unpaid Principal Balance | $ 59,562 | $ 49,391
Recorded Investment with Allowance | 48,301 | 39,688
Related Allowance | 6,023 | 5,647
Average Recorded Investment | 44,187 | 35,181
Real Estate [Member] | Non-residential Real Estate Owner Occupied [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Unpaid Principal Balance | 7,580 | 2,011
Recorded Investment with Allowance | 7,507 | 1,945
Related Allowance | 296 | 141
Average Recorded Investment | 5,944 | 1,858
Real Estate [Member] | Non-residential Real Estate Other [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Unpaid Principal Balance | 10,137 | 10,323
Recorded Investment with Allowance | 9,941 | 10,240
Related Allowance | 532 | 496
Average Recorded Investment | 10,023 | 3,975
Real Estate [Member] | Residential Real Estate Permanent Mortgage [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Unpaid Principal Balance | 2,118 | 1,745
Recorded Investment with Allowance | 1,918 | 1,542
Related Allowance | 138 | 146
Average Recorded Investment | 1,835 | 1,440
Real Estate [Member] | Residential Real Estate All Other [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Unpaid Principal Balance | 6,906 | 5,837
Recorded Investment with Allowance | 6,611 | 5,549
Related Allowance | 2,372 | 2,135
Average Recorded Investment | 6,241 | 5,258
Commercial and Financial [Member] | Non-consumer Non-real Estate [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Unpaid Principal Balance | 19,526 | 18,101
Recorded Investment with Allowance | 12,407 | 11,158
Related Allowance | 2,448 | 2,412
Average Recorded Investment | 11,705 | 11,131
Consumer Non-real Estate [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Unpaid Principal Balance | 850 | 545
Recorded Investment with Allowance | 817 | 514
Related Allowance | 134 | 127
Average Recorded Investment | 783 | 541
Other Loans [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Unpaid Principal Balance | 2,991 | 6,092
Recorded Investment with Allowance | 2,608 | 5,595
Related Allowance | 99 | 178
Average Recorded Investment | 2,570 | 7,439
Acquired Loans [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Unpaid Principal Balance | 9,454 | 4,737
Recorded Investment with Allowance | 6,492 | 3,145
Related Allowance | 4 | 12
Average Recorded Investment | $ 5,086 | $ 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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