Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Loan commitments	$ 3,218.0	$ 3,144.0
Fixed rate loan commitment	2,986.0
Floating rate loan commitment	232.0
Standby letters of credit	$ 85.7	68.7
Loan commitments floating rate, minimum	0.99%
Loan commitments floating rate, maximum	15.00%
Allowance for unfunded loan commitments	$ 3.2	3.1
Extended credit	12.1	12.4
Credit extensions with collateral	$ 9.2	$ 11.5