Commitments and Contingencies - Summary of Financial Instruments Outstanding (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Commitments to extend credit	$ 690,267	$ 642,162
Unfunded commitments under lines of credit	7,149,479	6,883,963
Commercial and standby letters of credit	246,622	240,436
Reserve for unfunded lending commitments	$ 15,281	$ 14,830	$ 14,433	$ 13,208