Loans - Summary of Recorded Investments in TDRs by Class of Loan (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2020 USD ($) contract	Mar. 31, 2019 USD ($) contract
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	54	62
Pre-Modification Outstanding Recorded Investment	$ 24.1	$ 19.4
Post-Modification Outstanding Recorded Investment	$ 24.1	$ 19.4
Commercial
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	24	20
Pre-Modification Outstanding Recorded Investment	$ 15.8	$ 11.7
Post-Modification Outstanding Recorded Investment	$ 15.8	$ 11.7
Retail
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	30	42
Pre-Modification Outstanding Recorded Investment	$ 8.3	$ 7.7
Post-Modification Outstanding Recorded Investment	$ 8.3	$ 7.7
Commercial real estate | Commercial
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	5	2
Pre-Modification Outstanding Recorded Investment	$ 3.7	$ 3.0
Post-Modification Outstanding Recorded Investment	$ 3.7	$ 3.0
Commercial real estate | Commercial | Extended Maturity
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	4
Post-Modification Outstanding Recorded Investment	$ 3.2
Commercial real estate | Commercial | Payment Deferral
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract		2
Post-Modification Outstanding Recorded Investment	$ 0.5	$ 3.0
Commercial and industrial | Commercial
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	10	4
Pre-Modification Outstanding Recorded Investment	$ 9.0	$ 1.4
Post-Modification Outstanding Recorded Investment	$ 9.0	$ 1.4
Commercial and industrial | Commercial | Extended Maturity
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	5	4
Post-Modification Outstanding Recorded Investment	$ 7.4	$ 1.4
Commercial and industrial | Commercial | Payment Deferral
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	1
Post-Modification Outstanding Recorded Investment	$ 0.3
Commercial and industrial | Commercial | Combination of Concessions
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	4
Post-Modification Outstanding Recorded Investment	$ 1.3
Equipment financing | Commercial
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	9	14
Pre-Modification Outstanding Recorded Investment	$ 3.1	$ 7.3
Post-Modification Outstanding Recorded Investment	$ 3.1	$ 7.3
Equipment financing | Commercial | Extended Maturity
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	1	8
Post-Modification Outstanding Recorded Investment	$ 1.2	$ 6.6
Equipment financing | Commercial | Payment Deferral
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	5	5
Post-Modification Outstanding Recorded Investment	$ 0.5	$ 0.5
Equipment financing | Commercial | Combination of Concessions
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	3	1
Post-Modification Outstanding Recorded Investment	$ 1.4	$ 0.2
MWABL | Commercial
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	0
Pre-Modification Outstanding Recorded Investment	$ 0.0
Post-Modification Outstanding Recorded Investment	$ 0.0
Residential mortgage | Retail
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	13	20
Pre-Modification Outstanding Recorded Investment	$ 6.2	$ 5.6
Post-Modification Outstanding Recorded Investment	$ 6.2	$ 5.6
Residential mortgage | Retail | Payment Deferral
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	9	5
Post-Modification Outstanding Recorded Investment	$ 5.5	$ 2.2
Residential mortgage | Retail | Combination of Concessions
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	1	6
Post-Modification Outstanding Recorded Investment	$ 0.1	$ 2.3
Residential mortgage | Retail | Bankruptcy
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	3	9
Post-Modification Outstanding Recorded Investment	$ 0.6	$ 1.1
Home equity | Retail
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	17	22
Pre-Modification Outstanding Recorded Investment	$ 2.1	$ 2.1
Post-Modification Outstanding Recorded Investment	$ 2.1	$ 2.1
Home equity | Retail | Payment Deferral
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	1	3
Post-Modification Outstanding Recorded Investment	$ 0.1	$ 0.5
Home equity | Retail | Combination of Concessions
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	11	10
Post-Modification Outstanding Recorded Investment	$ 1.5	$ 1.0
Home equity | Retail | Bankruptcy
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	5	9
Post-Modification Outstanding Recorded Investment	$ 0.5	$ 0.6
Other consumer | Retail
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	0	0
Pre-Modification Outstanding Recorded Investment	$ 0.0	$ 0.0
Post-Modification Outstanding Recorded Investment	$ 0.0	$ 0.0