Variable Interest Entities - Summary of Activity in Allowance for Loan Losses (Detail) - Variable Interest Entities Primary Beneficiary - Residential Mortgage Backed Securities - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Variable Interest Entity [Line Items]
Balance at beginning of period	$ 186	$ 203	$ 203
Charge-offs, net	(11)	(17)	(17)
Balance at end of period	$ 175	$ 186	$ 186