Special Purpose Entities, Including Variable Interest Entities - Supplemental Information about Loans Held as Assets and Long-Term Debt Associated with Consolidated Variable Interest Entities (Detail) - Variable Interest Entities [Member] - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Variable Interest Entities [Line Items]
Loans Unpaid Principal Balance	$ 2,478,571	$ 3,418,044
Long Term Debt Unpaid Principal Balance	$ 3,576,046	$ 4,552,643