Loans and Allowance for Loan Losses - Summary of Aging of Loans (Parenthetical) (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Receivables [Abstract]
loan premiums	$ 7,956	$ 9,047
capitalized loan origination costs	$ 15,729	$ 15,047