Other Assets and Liabilities - Summary of Components of Other Assets (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Accrued interest receivable	$ 34,715		$ 32,871
Collateral deposited under derivative agreements	7,750		79
Derivative assets	1,628	$ 1,628	4,086
Loan portfolio payments held by servicer	845		5,765
Prepaid expenses	469		803
Prepaid taxes	34		16
Other	239		450
Total	$ 45,680		$ 44,070