Commitments, Contingencies and Concentrations of Credit Risk - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Other Commitments [Line Items]
Fixed-rate loan commitments	90 days
Rental expense	$ 5.2	$ 3.2	$ 3.1
Minimum
Other Commitments [Line Items]
Interest rates	3.625%
Maximum
Other Commitments [Line Items]
Interest rates	7.50%