Loans Held for Sale, at Fair Value - Summary of Loans Held for Sale at Cost Basis and Fair Value (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Accounts Notes And Loans Receivable [Line Items]
Loans held for sale	$ 965,639	$ 362,635
Cost Basis
Accounts Notes And Loans Receivable [Line Items]
Loans held for sale	950,514	360,440
Fair Value
Accounts Notes And Loans Receivable [Line Items]
Loans held for sale	$ 965,639	$ 362,635