Fair Value Accounting - Assets Measured at Fair Value on Nonrecurring Basis (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans with an allowance recorded	$ (9.5)
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans with an allowance recorded		$ (8.9)
Fair Value, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets acquired through foreclosure	4.2	1.4
Collateral Dependent Loans, Net of Allowance	183.0	187.3
Fair Value, Nonrecurring [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets acquired through foreclosure	0.0	0.0
Collateral Dependent Loans, Net of Allowance	0.0	0.0
Fair Value, Nonrecurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets acquired through foreclosure	0.0	0.0
Collateral Dependent Loans, Net of Allowance	0.0	0.0
Fair Value, Nonrecurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets acquired through foreclosure	4.2	1.4
Collateral Dependent Loans, Net of Allowance	$ 183.0	$ 187.3
Loans and Finance Receivables [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Measurements, Valuation Processes, Description	Discounted cash flow method	Discounted cash flow method
Other Assets [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Measurements, Valuation Processes, Description	Collateral method	Collateral method
Collateral Method [Member] | Loans and Finance Receivables [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Measurements, Valuation Processes, Description	Collateral method	Collateral method
Third Party Appraisal [Member] | Loans and Finance Receivables [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Measurements, Valuation Processes, Description	Third party appraisal	Third party appraisal
Level 3 Fair Value Measurement Inputs	Costs to sell	Costs to sell
Third Party Appraisal [Member] | Other Assets [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Measurements, Valuation Processes, Description	Third party appraisal	Third party appraisal
Level 3 Fair Value Measurement Inputs	Costs to sell	Costs to sell
Discount Rate [Member] | Loans and Finance Receivables [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Measurements, Valuation Processes, Description	Discount rate	Discount rate
Level 3 Fair Value Measurement Inputs	Contractual loan rate	Contractual loan rate