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Fair Value Measurement - Additional Information (Detail) - non-recoverable rate [Member] - Level 3 - Appraisal Value
Sep. 30, 2018
Dec. 31, 2017
Minimum    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Loans held for investment, measurement input 0 0
Maximum    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Loans held for investment, measurement input 0.51 0.77
Weighted Average    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Loans held for investment, measurement input 0.27 0.21