Allowance for Credit Losses on Loans and Leases - Changes in Allowance for Credit Losses, Pre Adoption of ASU 326 (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period		$ 196.0	$ 180.0
Charge- Offs			(42.0)
Recoveries			14.0
Net Charge- Offs			(28.0)
Provision for credit losses	$ 1.0	123.0	44.0
Balance at End of Period			196.0
Acquired loans in business combination
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period		7.0	7.0
Charge- Offs			(9.0)
Recoveries			2.0
Net Charge- Offs			(7.0)
Provision for credit losses			7.0
Balance at End of Period			7.0
Total commercial loans and leases
Allowance for Loan and Lease Losses [Roll Forward]
Charge- Offs	(27.0)	(68.0)
Recoveries	15.1	15.0
Net Charge- Offs	(11.9)	(53.0)
Provision for credit losses	(7.3)	119.0
Total commercial loans and leases | Originated loans And leases
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period		126.0	114.0
Charge- Offs			(17.0)
Recoveries			8.0
Net Charge- Offs			(9.0)
Provision for credit losses			21.0
Balance at End of Period			126.0
Total commercial loans and leases | Commercial real estate
Allowance for Loan and Lease Losses [Roll Forward]
Charge- Offs	(8.8)	(31.0)
Recoveries	6.3	7.0
Net Charge- Offs	(2.5)	(24.0)
Provision for credit losses	(21.5)	67.0
Total commercial loans and leases | Commercial real estate | Originated loans And leases
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period		60.0	55.0
Charge- Offs			(4.0)
Recoveries			4.0
Net Charge- Offs			0.0
Provision for credit losses			5.0
Balance at End of Period			60.0
Total commercial loans and leases | Commercial and industrial
Allowance for Loan and Lease Losses [Roll Forward]
Charge- Offs	(15.7)	(32.0)
Recoveries	6.6	7.0
Net Charge- Offs	(9.1)	(25.0)
Provision for credit losses	15.3	41.0
Total commercial loans and leases | Commercial and industrial | Originated loans And leases
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period		53.0	49.0
Charge- Offs			(10.0)
Recoveries			4.0
Net Charge- Offs			(6.0)
Provision for credit losses			10.0
Balance at End of Period			53.0
Total commercial loans and leases | Commercial leases
Allowance for Loan and Lease Losses [Roll Forward]
Charge- Offs	(0.2)	(1.0)
Recoveries	0.9	0.0
Net Charge- Offs	0.7	(1.0)
Provision for credit losses	(3.3)	7.0
Total commercial loans and leases | Commercial leases | Originated loans And leases
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period		11.0	8.0
Charge- Offs			0.0
Recoveries			0.0
Net Charge- Offs			0.0
Provision for credit losses			3.0
Balance at End of Period			11.0
Total commercial loans and leases | Other
Allowance for Loan and Lease Losses [Roll Forward]
Charge- Offs	(2.3)	(4.0)
Recoveries	1.3	1.0
Net Charge- Offs	(1.0)	(3.0)
Provision for credit losses	2.2	4.0
Total commercial loans and leases | Other | Originated loans And leases
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period		2.0	2.0
Charge- Offs			(3.0)
Recoveries			0.0
Net Charge- Offs			(3.0)
Provision for credit losses			3.0
Balance at End of Period			2.0
Total consumer loans
Allowance for Loan and Lease Losses [Roll Forward]
Charge- Offs	(7.2)	(13.0)
Recoveries	5.2	6.0
Net Charge- Offs	(2.0)	(7.0)
Provision for credit losses	2.4	3.0
Total consumer loans | Originated loans And leases
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period		63.0	59.0
Charge- Offs			(16.0)
Recoveries			4.0
Net Charge- Offs			(12.0)
Provision for credit losses			16.0
Balance at End of Period			63.0
Total consumer loans | Direct installment
Allowance for Loan and Lease Losses [Roll Forward]
Charge- Offs	(1.4)	(1.0)
Recoveries	1.0	1.0
Net Charge- Offs	(0.4)	0.0
Provision for credit losses	0.8	2.0
Total consumer loans | Direct installment | Originated loans And leases
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period		13.0	14.0
Charge- Offs			(1.0)
Recoveries			0.0
Net Charge- Offs			(1.0)
Provision for credit losses			0.0
Balance at End of Period			13.0
Total consumer loans | Residential mortgages
Allowance for Loan and Lease Losses [Roll Forward]
Charge- Offs	(1.0)	(2.0)
Recoveries	0.6	1.0
Net Charge- Offs	(0.4)	(1.0)
Provision for credit losses	(0.2)	3.0
Total consumer loans | Residential mortgages | Originated loans And leases
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period		22.0	20.0
Charge- Offs			(2.0)
Recoveries			0.0
Net Charge- Offs			(2.0)
Provision for credit losses			4.0
Balance at End of Period			22.0
Total consumer loans | Indirect installment
Allowance for Loan and Lease Losses [Roll Forward]
Charge- Offs	(3.1)	(8.0)
Recoveries	2.2	4.0
Net Charge- Offs	(0.9)	(4.0)
Provision for credit losses	3.2	(6.0)
Total consumer loans | Indirect installment | Originated loans And leases
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period		19.0	15.0
Charge- Offs			(11.0)
Recoveries			4.0
Net Charge- Offs			(7.0)
Provision for credit losses			11.0
Balance at End of Period			19.0
Total consumer loans | Consumer lines of credit
Allowance for Loan and Lease Losses [Roll Forward]
Charge- Offs	(1.7)	(2.0)
Recoveries	1.4	0.0
Net Charge- Offs	(0.3)	(2.0)
Provision for credit losses	(1.4)	4.0
Total consumer loans | Consumer lines of credit | Originated loans And leases
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period		9.0	10.0
Charge- Offs			(2.0)
Recoveries			0.0
Net Charge- Offs			(2.0)
Provision for credit losses			1.0
Balance at End of Period			9.0
Allowance for credit losses on loans and leases
Allowance for Loan and Lease Losses [Roll Forward]
Charge- Offs	(34.2)	(81.0)
Recoveries	20.3	21.0
Net Charge- Offs	(13.9)	(59.8)
Provision for credit losses	$ (4.9)	121.8
Allowance for credit losses on loans and leases | Originated loans And leases
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period		$ 189.0	173.0
Charge- Offs			(33.0)
Recoveries			12.0
Net Charge- Offs			(21.0)
Provision for credit losses			37.0
Balance at End of Period			$ 189.0