Loans Held for Sale, at Fair Value - Summary of Loans Held for Sale at Cost Basis and Fair Value (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Accounts Notes And Loans Receivable [Line Items]
Loans held for sale	$ 704,357	$ 990,864
Cost Basis [Member]
Accounts Notes And Loans Receivable [Line Items]
Cost Basis	681,835	972,434
Fair Value [Member]
Accounts Notes And Loans Receivable [Line Items]
Loans held for sale	$ 704,357	$ 990,864