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, 2018	Dec. 31, 2017
Loans Receivable Held For Sale Net [Abstract]
Cost basis	$ 1,132,205	$ 360,440
Fair Value	$ 1,132,665	$ 362,635