Commitments and Contingencies - Summary of Financial Instruments Outstanding (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Commitments and Contingencies Disclosure [Abstract]
Commitments to grant loans	$ 419,847	$ 355,558
Unfunded commitments under lines of credit	4,912,092	4,899,930
Commercial and standby letters of credit	165,784	163,560
Reserve for unfunded lending commitments	$ 11,660	$ 11,241	$ 14,033	$ 14,145