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Fair Value Measurements - Narrative (Detail) - USD ($)
3 Months Ended 12 Months Ended
Mar. 31, 2019
Dec. 31, 2018
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