Allowance for Credit Losses - Summary of Allowance for Credit Losses (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Receivables [Abstract]
Allowance for loan losses	$ 161,759	$ 151,859	$ 120,856
Allowance for unfunded letters of credit	2,845	4,436
Total allowance for credit losses	$ 164,604	$ 156,295