Loan Servicing - SBA-Guaranteed Loan Servicing Asset Activity (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Servicing Assets at Fair Value [Line Items]
SBA service fees	$ 122.0	$ 108.0	$ 124.0
Indirect Auto Loan
Servicing Assets at Fair Value [Line Items]
SBA loans sold with servicing retained		500.0
Servicing Asset at Fair Value, Amount [Roll Forward]
Loan servicing rights	0.3	0.7
SBA-Guaranteed Loan Servicing
Servicing Assets at Fair Value [Line Items]
SBA loans sold to investors with servicing retained	230.0	217.0
SBA loans sold with servicing retained	64.0	33.0	23.0
Pretax gains resulting from above loan sales	8.0	3.0	2.0
SBA service fees	2.0	2.0	2.0
Servicing Asset at Fair Value, Amount [Roll Forward]
Balance at beginning of period	3.0	3.0	4.0
Additions	1.0	1.0	0.0
Payoffs, curtailments and amortization	(1.0)	(1.0)	(1.0)
Balance at end of period	3.0	3.0	3.0
Fair value, beginning of period	3.0	3.0	4.0
Fair value, end of period	3.0	3.0	$ 3.0
Valuation allowance for impairment of recognized servicing assets, balance	$ 1.1	$ 1.1