Loans - Summary of Recorded Investments in TDRs by Class of Loan, Subsequently Defaulted (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	9	7
Recorded Investment as of Period End | $	$ 3.9	$ 4.6
Commercial
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	5	2
Recorded Investment as of Period End | $	$ 2.6	$ 2.9
Commercial | Commercial real estate
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	0	0
Recorded Investment as of Period End | $	$ 0.0	$ 0.0
Commercial | Commercial and industrial
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	1	0
Recorded Investment as of Period End | $	$ 0.1	$ 0.0
Commercial | Equipment financing
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	4	2
Recorded Investment as of Period End | $	$ 2.5	$ 2.9
Commercial | MWABL
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	0	0
Recorded Investment as of Period End | $	$ 0.0	$ 0.0
Retail
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	4	5
Recorded Investment as of Period End | $	$ 1.3	$ 1.7
Retail | Residential mortgage
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	3	3
Recorded Investment as of Period End | $	$ 1.3	$ 1.3
Retail | Home equity
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	1	2
Recorded Investment as of Period End | $	$ 0.0	$ 0.4
Retail | Other consumer
Financing Receivable, Modifications [Line Items]
Number of Contracts | contract	0	0
Recorded Investment as of Period End | $	$ 0.0	$ 0.0