Other Assets and Liabilities - Summary of Components of Other Assets (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Accrued interest receivable	$ 39,926	$ 38,573
Collateral deposited under derivative agreements	6,590	4,120
Loan portfolio payments held by servicer	4,750	54,759
Derivative assets	3,119	1,214
Prepaid expenses	608	798
Prepaid taxes	28	31
Other	75	80
Total	$ 55,096	$ 99,575