Other Assets and Liabilities - Summary of Components of Other Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Debt securities held-to-maturity	$ 96,167
Accrued interest receivable	56,679	$ 38,573
Derivative assets	9,916	1,214
Loan portfolio payments held by servicer	6,133	54,759
Prepaid expenses	647	798
Prepaid taxes	6	31
Collateral deposited under derivative agreements		4,120
Other	965	80
Total	$ 170,513	$ 99,575