Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Commitments and Contingencies Disclosure [Abstract]
Loan commitments	$ 3,144.0	$ 2,354.0
Fixed rate loan commitment	2,896.0
Floating rate loan commitment	248.1
Standby letters of credit	$ 68.7	51.7
Loan commitments floating rate, minimum	0.00%
Loan commitments floating rate, maximum	18.00%
Allowance for unfunded loan commitments	$ 3.1	3.2
Extended credit	12.4	13.3
Credit extensions with collateral	$ 11.5	$ 12.6