Allowance for Credit Losses on Loans and Leases - Changes in Allowance for Credit Losses (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Year	$ 363.0
Charge- Offs			$ (42.0)
Recoveries			14.0
Net Charge- Offs			(28.0)
Provision for credit losses	1.0	$ 123.0	44.0
Allowance for credit loss			196.0	$ 180.0
Balance at End of Year	344.0	363.0
Unfunded loan commitment
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Year	13.6	3.0
Charge- Offs	0.0	0.0
Recoveries	0.0	0.0
Net Charge- Offs	0.0	0.0
Provision for credit losses	5.5	1.0
Initial ACL on PCD Loans			0.0
Balance at End of Year	19.1	13.6	3.0
Unfunded loan commitment | Impact of ASC 326 Adoption
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit loss			10.0
Total commercial loans and leases
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Year	280.4	133.0
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
Initial ACL on PCD Loans			44.0
Balance at End of Year	261.2	280.4	133.0
Total commercial loans and leases | Impact of ASC 326 Adoption
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit loss			37.0
Total commercial loans and leases | Commercial real estate
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Year	180.5	60.0
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
Initial ACL on PCD Loans			40.0
Balance at End of Year	156.5	180.5	60.0
Total commercial loans and leases | Commercial real estate | Impact of ASC 326 Adoption
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit loss			38.0
Total commercial loans and leases | Commercial and industrial
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Year	81.2	53.0
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
Initial ACL on PCD Loans			4.0
Balance at End of Year	87.4	81.2	53.0
Total commercial loans and leases | Commercial and industrial | Impact of ASC 326 Adoption
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit loss			8.0
Total commercial loans and leases | Commercial leases
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Year	17.3	11.0
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
Initial ACL on PCD Loans			0.0
Balance at End of Year	14.7	17.3	11.0
Total commercial loans and leases | Commercial leases | Impact of ASC 326 Adoption
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit loss			0.0
Total commercial loans and leases | Other
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Year	1.4	9.0
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
Initial ACL on PCD Loans			0.0
Balance at End of Year	2.6	1.4	9.0
Total commercial loans and leases | Other | Impact of ASC 326 Adoption
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit loss			(9.0)
Total commercial loans and leases | Unfunded loan commitment
Allowance for Loan and Lease Losses [Roll Forward]
Provision for credit losses	5.5	1.0
Total commercial loans and leases | Unfunded loan commitment | Impact of ASC 326 Adoption
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit loss			8.0
Total consumer loans
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Year	82.7	63.0
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
Initial ACL on PCD Loans			6.0
Balance at End of Year	83.1	82.7	63.0
Total consumer loans | Impact of ASC 326 Adoption
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit loss			18.0
Total consumer loans | Direct installment
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Year	26.0	13.0
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
Initial ACL on PCD Loans			1.0
Balance at End of Year	26.4	26.0	13.0
Total consumer loans | Direct installment | Impact of ASC 326 Adoption
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit loss			10.0
Total consumer loans | Residential mortgages
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Year	33.7	22.0
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
Initial ACL on PCD Loans			4.0
Balance at End of Year	33.1	33.7	22.0
Total consumer loans | Residential mortgages | Impact of ASC 326 Adoption
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit loss			6.0
Total consumer loans | Indirect installment
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Year	11.2	19.0
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)
Initial ACL on PCD Loans			0.0
Balance at End of Year	13.5	11.2	19.0
Total consumer loans | Indirect installment | Impact of ASC 326 Adoption
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit loss			2.0
Total consumer loans | Consumer lines of credit
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Year	11.8	9.0
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
Initial ACL on PCD Loans			1.0
Balance at End of Year	10.1	11.8	9.0
Total consumer loans | Consumer lines of credit | Impact of ASC 326 Adoption
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit loss			0.0
Total consumer loans | Unfunded loan commitment
Allowance for Loan and Lease Losses [Roll Forward]
Provision for credit losses	0.0	0.0
Total consumer loans | Unfunded loan commitment | Impact of ASC 326 Adoption
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit loss			2.0
Allowance for credit losses on loans and leases
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Year	363.1	196.0
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
Initial ACL on PCD Loans			50.0
Balance at End of Year	344.3	363.1	196.0
Allowance for credit losses on loans and leases | Impact of ASC 326 Adoption
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit loss			55.0
Allowance for credit losses on loans and leases and allowance for unfunded loan commitments
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Year	376.7	199.0
Charge- Offs	(34.2)	(81.0)
Recoveries	20.3	21.0
Net Charge- Offs	(13.9)	(60.0)
Provision for credit losses	0.6	123.0
Initial ACL on PCD Loans			50.0
Balance at End of Year	$ 363.4	$ 376.7	199.0
Allowance for credit losses on loans and leases and allowance for unfunded loan commitments | Impact of ASC 326 Adoption
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit loss			$ 65.0