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Loans Held for Sale, at Fair Value - Additional Information (Detail) - USD ($)
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Dec. 31, 2018
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
Management Services, Servicing Fees and Other [Member]          
Accounts Notes And Loans Receivable [Line Items]          
Interest income on Loans held for sale 6,500,000 $ 5,000,000.0 15,100,000 $ 9,700,000  
Gains (Loss) from Mortgage Banking Activities, Net [Member]          
Accounts Notes And Loans Receivable [Line Items]          
Gains (loss) recognized for fair value adjustments on loans held for sale 12,400,000 $ (7,800,000) 25,700,000 $ 7,300,000  
Greater Than 90 Days Past Due [Member]          
Accounts Notes And Loans Receivable [Line Items]          
Loans held for sale past due $ 0   $ 0   $ 0