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Loans Held for Sale, at Fair Value - Additional Information (Detail) - USD ($)
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Accounts Notes And Loans Receivable [Line Items]          
Maximum number of days loans held for sale are typically sold     45 days    
Loans held for sale in nonaccrual status $ 0   $ 0   $ 0
Recognized gain(losses) from change in fair value on loans held for sale (6,800,000) $ 600,000 500,000 $ 500,000  
Interest Income [Member]          
Accounts Notes And Loans Receivable [Line Items]          
Interest income on Loans held for sale 10,300,000 $ 5,700,000 19,900,000 $ 24,900,000  
Greater Than 90 Days Past Due [Member]          
Accounts Notes And Loans Receivable [Line Items]          
Loans held for sale past due $ 0   $ 0   $ 0