Commitments and Contingencies - Summary of Financial Instruments Outstanding (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Commitments and Contingencies Disclosure [Abstract]
Commitments to extend credit	$ 824,472	$ 342,305
Unfunded commitments under lines of credit	6,610,843	6,060,034
Commercial and standby letters of credit	235,730	210,002
Reserve for unfunded lending commitments	$ 14,721	$ 13,208	$ 21,032	$ 11,241