Allowance for Credit Losses - Provision for Credit Losses (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Receivables [Abstract]
Provision for loan losses	$ 10,702	$ 2,402
Provision for unfunded letters of credit	246	68
Total provision for credit losses	$ 10,948	$ 2,470