Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Commitments and Contingencies Disclosure [Abstract]
Loan commitments	$ 2,588.0	$ 2,354.0
Fixed rate loan commitment	2,425.0
Floating rate loan commitment	163.1
Standby letters of credit	$ 53.0	51.7
Loan commitments floating rate, minimum	0.00%
Loan commitments floating rate, maximum	21.00%
Allowance for unfunded loan commitments	$ 2.6	3.2
Extended credit	12.8	13.3
Credit extensions with collateral	$ 12.4	$ 12.6