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Loans and Allowance for Credit Losses - Additional Information (Detail) | 12 Months Ended
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Dec. 31, 2017 USD ($) Contract | Dec. 31, 2016 USD ($) Contract | Dec. 31, 2015 USD ($) Contract
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Provision for credit losses | $ 5,087,000 | $ 18,480,000 | $ 14,948,000
Charge-offs |  |  | $ 18,868,000
Number of loans modified | Contract | 41 | 65 | 61
Total amount of loan modified | $ 8,274,000 | $ 10,275,000 | $ 10,956,000
Off balance sheet reserve to nonperforming loans |  | 12,000 |
Loans held for sale | 14,850,000 | 7,052,000 |
Total gains or losses included in earnings | 4,329,000 | 589,000 |
Loans with modification to rate and payment due to amortization | $ 300,000 | 6,600,000 | $ 3,700,000
Minimum days required for conversion of trouble debt to be in default | 90 days |  |
Impaired Loans [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Unfunded commitments related to nonperforming loans | $ 2,400,000 | 1,779,000 |
Consumer Loan [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Non Accrual Status Of Loans After Number Of Days Past Due | 150 days |  |
Nonaccrual Commercial Loans [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Gain (loss) on sale of impaired loans | $ 21,000 | $ 0 |
Reserve for Off-balance Sheet Activities [Member] | Impaired Loans [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Off balance sheet reserve to nonperforming loans | $ 200,000 |  |

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- Definition Unfunded commitments related to nonperforming loans. + References No definition available. + Details
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- Definition Amount of gain (loss) recognized in the income statement for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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- Definition Amount of direct write-downs of financing receivables charged against the allowance. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524 + Details
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- Definition The gains (losses) included in earnings that represent the difference between the sale price and the carrying value of loans and leases that were sold during the reporting period. This element refers to the gain (loss) and not to the cash proceeds of the sales. This element is a noncash adjustment to net income when calculating net cash generated by operating activities using the indirect method. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 24 -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3521-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585 + Details
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- Definition Amount, after allowance for bad debt, of loan receivables held-for-sale that are not part of a disposal group. Excludes loans and leases covered under loss sharing agreements and loans classified as debt securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=82891408&loc=d3e4428-111522 + Details
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