Loans - Summary of Credit Quality Indicators by Class of Loan (Detail) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Financing Receivable, Recorded Investment [Line Items]
Loans	$ 44,284.0	$ 43,596.1
Commercial
Financing Receivable, Recorded Investment [Line Items]
Loans	31,710.0	30,714.3
Retail
Financing Receivable, Recorded Investment [Line Items]
Loans	12,574.0	12,881.8
Originated | Commercial | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans		29,047.5
Originated | Commercial | Special mention
Financing Receivable, Recorded Investment [Line Items]
Loans		741.2
Originated | Commercial | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans		918.8
Originated | Commercial | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans		6.8
Originated | Retail | Low risk
Financing Receivable, Recorded Investment [Line Items]
Loans		5,182.2
Originated | Retail | Moderate risk
Financing Receivable, Recorded Investment [Line Items]
Loans		5,512.7
Originated | Retail | High risk
Financing Receivable, Recorded Investment [Line Items]
Loans		2,186.9
Commercial real estate | Commercial
Financing Receivable, Recorded Investment [Line Items]
Loans	14,651.6	14,762.3
Commercial real estate | Originated | Commercial | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans		14,222.3
Commercial real estate | Originated | Commercial | Special mention
Financing Receivable, Recorded Investment [Line Items]
Loans		334.3
Commercial real estate | Originated | Commercial | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans		202.6
Commercial real estate | Originated | Commercial | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans		3.1
Commercial and industrial | Commercial
Financing Receivable, Recorded Investment [Line Items]
Loans	9,097.7	8,693.2
Commercial and industrial | Commercial
Financing Receivable, Recorded Investment [Line Items]
Loans	12,045.7	11,041.6
Commercial and industrial | Originated | Commercial | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans		10,391.7
Commercial and industrial | Originated | Commercial | Special mention
Financing Receivable, Recorded Investment [Line Items]
Loans		330.4
Commercial and industrial | Originated | Commercial | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans		315.8
Commercial and industrial | Originated | Commercial | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans		3.7
Equipment financing | Commercial
Financing Receivable, Recorded Investment [Line Items]
Loans	5,012.7	4,910.4
Equipment financing | Originated | Commercial | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans		4,433.5
Equipment financing | Originated | Commercial | Special mention
Financing Receivable, Recorded Investment [Line Items]
Loans		76.5
Equipment financing | Originated | Commercial | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans		400.4
Equipment financing | Originated | Commercial | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans		0.0
Residential mortgage | Retail
Financing Receivable, Recorded Investment [Line Items]
Loans	10,081.9	10,318.1
Residential mortgage | Originated | Retail | Low risk
Financing Receivable, Recorded Investment [Line Items]
Loans		4,410.0
Residential mortgage | Originated | Retail | Moderate risk
Financing Receivable, Recorded Investment [Line Items]
Loans		4,958.3
Residential mortgage | Originated | Retail | High risk
Financing Receivable, Recorded Investment [Line Items]
Loans		949.8
Home equity | Retail
Financing Receivable, Recorded Investment [Line Items]
Loans	2,349.9	2,406.5
Home equity | Originated | Retail | Low risk
Financing Receivable, Recorded Investment [Line Items]
Loans		747.3
Home equity | Originated | Retail | Moderate risk
Financing Receivable, Recorded Investment [Line Items]
Loans		548.5
Home equity | Originated | Retail | High risk
Financing Receivable, Recorded Investment [Line Items]
Loans		1,110.7
Other consumer | Retail
Financing Receivable, Recorded Investment [Line Items]
Loans	$ 142.2	157.2
Other consumer | Originated | Retail | Low risk
Financing Receivable, Recorded Investment [Line Items]
Loans		24.9
Other consumer | Originated | Retail | Moderate risk
Financing Receivable, Recorded Investment [Line Items]
Loans		5.9
Other consumer | Originated | Retail | High risk
Financing Receivable, Recorded Investment [Line Items]
Loans		$ 126.4