Loans - Summary of Aging Information by Class of Loan (Detail) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	$ 43,881.4	$ 43,273.2
Total Past Due	402.6	322.9
Total loans	44,284.0	43,596.1
30-89 Days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	277.3	209.8
90 Days or More
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	125.3	113.1
Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	31,436.6	30,515.5
Total Past Due	273.4	198.8
Total loans	31,710.0	30,714.3
Commercial | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	14,580.0	14,713.8
Total Past Due	71.6	48.5
Total loans	14,651.6	14,762.3
Commercial | Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	9,055.4
Total Past Due	42.3
Total loans	9,097.7	8,693.2
Commercial | Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current		11,010.7
Total Past Due		30.9
Total loans	12,045.7	11,041.6
Commercial | Equipment financing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	4,853.2	4,791.0
Total Past Due	159.5	119.4
Total loans	5,012.7	4,910.4
Commercial | MWABL
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	2,948.0
Total Past Due	0.0
Total loans	2,948.0	2,348.4
Commercial | 30-89 Days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	198.9	133.3
Commercial | 30-89 Days | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	43.7	22.3
Commercial | 30-89 Days | Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	18.8
Commercial | 30-89 Days | Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		13.1
Commercial | 30-89 Days | Equipment financing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	136.4	97.9
Commercial | 30-89 Days | MWABL
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0.0
Commercial | 90 Days or More
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	74.5	65.5
Commercial | 90 Days or More | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	27.9	26.2
Commercial | 90 Days or More | Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	23.5
Commercial | 90 Days or More | Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		17.8
Commercial | 90 Days or More | Equipment financing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	23.1	21.5
Commercial | 90 Days or More | MWABL
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0.0
Retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	12,444.8	12,757.7
Total Past Due	129.2	124.1
Total loans	12,574.0	12,881.8
Retail | Residential mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	9,978.3	10,215.9
Total Past Due	103.6	102.2
Total loans	10,081.9	10,318.1
Retail | Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	2,325.3	2,385.7
Total Past Due	24.6	20.8
Total loans	2,349.9	2,406.5
Retail | Other consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	141.2	156.1
Total Past Due	1.0	1.1
Total loans	142.2	157.2
Retail | 30-89 Days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	78.4	76.5
Retail | 30-89 Days | Residential mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	64.2	65.5
Retail | 30-89 Days | Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	13.3	9.9
Retail | 30-89 Days | Other consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0.9	1.1
Retail | 90 Days or More
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	50.8	47.6
Retail | 90 Days or More | Residential mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	39.4	36.7
Retail | 90 Days or More | Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	11.3	10.9
Retail | 90 Days or More | Other consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	$ 0.1	$ 0.0