Other Assets and Liabilities - Summary of Components of Other Assets (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Debt securities held-to-maturity	$ 96,050
Accrued interest receivable	45,762	$ 38,573
Collateral deposited under derivative agreements	7,360	4,120
Loan portfolio payments held by servicer	3,902	54,759
Derivative assets	2,719	1,214
Prepaid expenses	151	798
Prepaid taxes	28	31
Other		80
Total	$ 155,972	$ 99,575