Commitments and Contingencies - Summary of Financial Instruments Outstanding (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Commitments and Contingencies Disclosure [Abstract]
Commitments to grant loans	$ 942,823	$ 355,558
Unfunded commitments under lines of credit	5,307,192	4,899,930
Commercial and standby letters of credit	211,413	163,560
Reserve for unfunded lending commitments	$ 21,032	$ 11,241	$ 11,990	$ 14,145