Loans - Summary of Credit Quality Indicators by Class of Loan (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	$ 43,596.1	$ 35,241.4
Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	30,714.3	25,077.7
Retail Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	12,881.8	10,163.7
Originated [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	24,481.7	22,028.5
Originated [Member] | Commercial [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	23,191.5	21,012.0
Originated [Member] | Commercial [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	568.9	394.5
Originated [Member] | Commercial [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	716.1	620.8
Originated [Member] | Commercial [Member] | Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	5.2	1.2
Originated [Member] | Retail Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	8,253.7	8,621.5
Originated [Member] | Retail Loans [Member] | Low Risk [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	3,997.5	3,774.6
Originated [Member] | Retail Loans [Member] | Moderate Risk [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	3,203.4	3,943.2
Originated [Member] | Retail Loans [Member] | High Risk [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	1,052.8	903.7
Acquired Loans [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	6,232.6	3,049.2
Acquired Loans [Member] | Commercial [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	5,856.0	2,879.8
Acquired Loans [Member] | Commercial [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	172.3	46.6
Acquired Loans [Member] | Commercial [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	202.7	122.8
Acquired Loans [Member] | Commercial [Member] | Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	1.6	0.0
Acquired Loans [Member] | Retail Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	4,628.1	1,542.2
Acquired Loans [Member] | Retail Loans [Member] | Low Risk [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	1,184.7	506.1
Acquired Loans [Member] | Retail Loans [Member] | Moderate Risk [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	2,309.3	639.6
Acquired Loans [Member] | Retail Loans [Member] | High Risk [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	1,134.1	396.5
Commercial Real Estate Loan [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	14,762.3	11,649.6
Commercial Real Estate Loan [Member] | Originated [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	10,012.5	9,798.5
Commercial Real Estate Loan [Member] | Originated [Member] | Commercial [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	9,736.4	9,607.0
Commercial Real Estate Loan [Member] | Originated [Member] | Commercial [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	197.6	105.5
Commercial Real Estate Loan [Member] | Originated [Member] | Commercial [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	75.4	85.2
Commercial Real Estate Loan [Member] | Originated [Member] | Commercial [Member] | Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	3.1	0.8
Commercial Real Estate Loan [Member] | Acquired Loans [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	4,749.8	1,851.1
Commercial Real Estate Loan [Member] | Acquired Loans [Member] | Commercial [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	4,485.9	1,766.2
Commercial Real Estate Loan [Member] | Acquired Loans [Member] | Commercial [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	136.7	27.3
Commercial Real Estate Loan [Member] | Acquired Loans [Member] | Commercial [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	127.2	57.6
Commercial Real Estate Loan [Member] | Acquired Loans [Member] | Commercial [Member] | Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	0.0	0.0
Commercial and Industrial [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	11,041.6	9,088.9
Commercial and Industrial [Member] | Originated [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	9,763.1	8,292.3
Commercial and Industrial [Member] | Originated [Member] | Commercial [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	9,223.5	7,855.7
Commercial and Industrial [Member] | Originated [Member] | Commercial [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	294.8	196.9
Commercial and Industrial [Member] | Originated [Member] | Commercial [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	242.7	239.3
Commercial and Industrial [Member] | Originated [Member] | Commercial [Member] | Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	2.1	0.4
Commercial and Industrial [Member] | Acquired Loans [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	1,278.5	796.6
Commercial and Industrial [Member] | Acquired Loans [Member] | Commercial [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	1,168.2	719.6
Commercial and Industrial [Member] | Acquired Loans [Member] | Commercial [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	35.6	14.6
Commercial and Industrial [Member] | Acquired Loans [Member] | Commercial [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	73.1	62.4
Commercial and Industrial [Member] | Acquired Loans [Member] | Commercial [Member] | Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	1.6	0.0
Equipment Financing [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	4,910.4	4,339.2
Equipment Financing [Member] | Originated [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	4,706.1	3,937.7
Equipment Financing [Member] | Originated [Member] | Commercial [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	4,231.6	3,549.3
Equipment Financing [Member] | Originated [Member] | Commercial [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	76.5	92.1
Equipment Financing [Member] | Originated [Member] | Commercial [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	398.0	296.3
Equipment Financing [Member] | Originated [Member] | Commercial [Member] | Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	0.0	0.0
Equipment Financing [Member] | Acquired Loans [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	204.3	401.5
Equipment Financing [Member] | Acquired Loans [Member] | Commercial [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	201.9	394.0
Equipment Financing [Member] | Acquired Loans [Member] | Commercial [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	0.0	4.7
Equipment Financing [Member] | Acquired Loans [Member] | Commercial [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	2.4	2.8
Equipment Financing [Member] | Acquired Loans [Member] | Commercial [Member] | Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	0.0	0.0
Residential Mortgage Loan [Member] | Retail Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	10,318.1	8,154.2
Residential Mortgage Loan [Member] | Originated [Member] | Retail Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	6,588.9	6,789.2
Residential Mortgage Loan [Member] | Originated [Member] | Retail Loans [Member] | Low Risk [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	3,225.3	2,912.8
Residential Mortgage Loan [Member] | Originated [Member] | Retail Loans [Member] | Moderate Risk [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	2,649.0	3,360.9
Residential Mortgage Loan [Member] | Originated [Member] | Retail Loans [Member] | High Risk [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	714.6	515.5
Residential Mortgage Loan [Member] | Acquired Loans [Member] | Retail Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	3,729.2	1,365.0
Residential Mortgage Loan [Member] | Acquired Loans [Member] | Retail Loans [Member] | Low Risk [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	1,184.7	506.1
Residential Mortgage Loan [Member] | Acquired Loans [Member] | Retail Loans [Member] | Moderate Risk [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	2,309.3	639.6
Residential Mortgage Loan [Member] | Acquired Loans [Member] | Retail Loans [Member] | High Risk [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	235.2	219.3
Home Equity Loan [Member] | Retail Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	2,406.5	1,962.5
Home Equity Loan [Member] | Originated [Member] | Retail Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	1,625.1	1,789.5
Home Equity Loan [Member] | Originated [Member] | Retail Loans [Member] | Low Risk [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	747.3	834.5
Home Equity Loan [Member] | Originated [Member] | Retail Loans [Member] | Moderate Risk [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	548.5	576.4
Home Equity Loan [Member] | Originated [Member] | Retail Loans [Member] | High Risk [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	329.3	378.6
Home Equity Loan [Member] | Acquired Loans [Member] | Retail Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	781.4	173.0
Home Equity Loan [Member] | Acquired Loans [Member] | Retail Loans [Member] | Low Risk [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	0.0	0.0
Home Equity Loan [Member] | Acquired Loans [Member] | Retail Loans [Member] | Moderate Risk [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	0.0	0.0
Home Equity Loan [Member] | Acquired Loans [Member] | Retail Loans [Member] | High Risk [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	781.4	173.0
Other Consumer [Member] | Retail Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	157.2	47.0
Other Consumer [Member] | Originated [Member] | Retail Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	39.7	42.8
Other Consumer [Member] | Originated [Member] | Retail Loans [Member] | Low Risk [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	24.9	27.3
Other Consumer [Member] | Originated [Member] | Retail Loans [Member] | Moderate Risk [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	5.9	5.9
Other Consumer [Member] | Originated [Member] | Retail Loans [Member] | High Risk [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	8.9	9.6
Other Consumer [Member] | Acquired Loans [Member] | Retail Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	117.5	4.2
Other Consumer [Member] | Acquired Loans [Member] | Retail Loans [Member] | Low Risk [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	0.0	0.0
Other Consumer [Member] | Acquired Loans [Member] | Retail Loans [Member] | Moderate Risk [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	0.0	0.0
Other Consumer [Member] | Acquired Loans [Member] | Retail Loans [Member] | High Risk [Member]
Financing Receivable, Recorded Investment [Line Items]
Financing Receivable	$ 117.5	$ 4.2