Commitments and Contingencies - Summary of Financial Instruments Outstanding (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Commitments and Contingencies Disclosure [Abstract]
Commitments to extend credit	$ 702,966	$ 342,305
Unfunded commitments under lines of credit	6,503,855	6,060,034
Commercial and standby letters of credit	213,341	210,002
Reserve for unfunded lending commitments	$ 14,433	$ 13,208	$ 10,462	$ 11,241