Other Assets and Liabilities - Summary of Components of Other Assets (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Debt securities held-to-maturity	$ 94,595	$ 96,167
Accrued interest receivable	51,065	56,679
Loan portfolio payments held by servicer	43,236	6,133
Derivative assets	14,000	9,916
Prepaid expenses	278	647
Prepaid taxes	3	6
Other	324	965
Total	$ 203,501	$ 170,513