Commitments and Contingencies - Summary of Unfunded Commitments (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Unfunded credit extension commitments	$ 1,710,962	$ 1,544,512
Commitments to fund loans
Line of Credit Facility [Line Items]
Unfunded credit extension commitments	985,837	926,405
Unused lines of credit
Line of Credit Facility [Line Items]
Unfunded credit extension commitments	674,149	571,587
Commercial and standby letters of credit
Line of Credit Facility [Line Items]
Unfunded credit extension commitments	$ 50,976	$ 46,520