Commitments, Contingencies and Concentrations of Credit Risk - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other Commitments [Line Items]
Fixed-rate loan commitments	90 days
Rental expense	$ 3.2	$ 3.1	$ 2.3
Minimum
Other Commitments [Line Items]
Interest rates	2.25%
Maximum
Other Commitments [Line Items]
Interest rates	5.99%