Commitments and Contingencies - Summary of Financial Instruments Outstanding (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Commitments to extend credit	$ 883,615	$ 642,162
Unfunded commitments under lines of credit	7,252,052	6,883,963
Commercial and standby letters of credit	215,664	240,436
Reserve for unfunded lending commitments	$ 16,144	$ 14,830	$ 14,721	$ 13,208