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Fair Value Measurements - Narrative (Detail) - USD ($) | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2018 | Dec. 31, 2017
Loans held for sale | Nonrecurring basis |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Non recurring fair value adjustments on loans held for sale | $ 0 | $ 0
Maximum |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Period under consideration for valuation of real estate collateral | 12 months |
Period under consideration for valuation of business equipment | 1 year |
Minimum |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Period under consideration for valuation of house or building in the process of construction | 1 year |

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