Commitments and Contingencies - Summary of Financial Instruments Outstanding (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Commitments to extend credit	$ 760,056	$ 642,162
Unfunded commitments under lines of credit	7,003,670	6,883,963
Commercial and standby letters of credit	241,516	240,436
Reserve for unfunded lending commitments	$ 15,981	$ 14,830	$ 13,432	$ 13,208