Commitments and Contingencies - Additional Information (Detail)	9 Months Ended
	Sep. 30, 2018 USD ($) shares	Dec. 31, 2017 USD ($)
Loss Contingencies [Line Items]
Loan commitments	$ 3,313,000,000	$ 3,144,000,000
Fixed rate loan commitment	3,154,000,000
Floating rate loan commitment	158,700,000
Standby letters of credit	$ 91,600,000	68,700,000
Loan commitments floating rate, minimum	0.99%
Loan commitments floating rate, maximum	15.25%
Allowance for unfunded loan commitments	$ 2,200,000	3,100,000
Extended credit	11,100,000	12,400,000
Credit extensions with collateral	$ 7,800,000	$ 11,500,000
Class B Restricted Shares [Member] | Visa [Member]
Loss Contingencies [Line Items]
Restricted stock conversion ratio	1.6298
Investment owned, balance, shares | shares	56,210
Investment owned, at cost	$ 0