Allowance for Credit Losses - Allowance for Credit Losses (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015
Receivables [Abstract]
Allowance for loan losses	$ 118,966	$ 116,446	$ 114,419	$ 110,697	$ 108,088	$ 106,178
Allowance for unfunded letters of credit	2,514		2,185
Total allowance for credit losses	$ 121,480		$ 116,604