Commitments and Contingencies - Summary of Financial Instruments Outstanding (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Commitments and Contingencies Disclosure [Abstract]
Commitments to grant loans	$ 319,318	$ 342,305
Unfunded commitments under lines of credit	6,340,135	6,060,034
Commercial and standby letters of credit	211,070	210,002
Reserve for unfunded lending commitments	$ 13,432	$ 13,208	$ 11,660	$ 11,241