Fair Value Accounting - Change in Level 3 Liabilities Measured at Fair Value on Recurring Basis (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other Comprehensive Income (Loss), Financial Liability, Fair Value Option, Unrealized Gain (Loss) Arising During Period, after Tax	$ (1.2)	$ (3.1)	$ (9.8)
Junior subordinated debt	(67.4)	(65.9)
Fair Value, Recurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Junior subordinated debt	$ (67.4)	$ (65.9)
Loans and Finance Receivables [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Junior subordinated debt, Significant Unobservable Inputs	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]
Junior subordinated debt, Significant Unobservable Inputs	Collateral method	Collateral method
Junior Subordinated Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Other Comprehensive Income (Loss)	$ (1.5)	$ (4.2)	(13.0)
Debt Instrument, Measurement Input	0.0261	0.0287
Debt Instrument, Basis Spread on Variable Rate	2.40%	2.64%
Percentage Of Basis Spread On Variable Rate	0.21%	0.24%
Junior Subordinated Debt [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Junior subordinated debt, Significant Unobservable Inputs	Implied credit rating of the Company	Implied credit rating of the Company
Junior subordinated debt	$ (67.4)	$ (65.9)
Junior Subordinated Debt [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning balance	(65.9)	(61.7)	48.7
Closing balance	$ (67.4)	$ (65.9)	$ (61.7)
Collateral Method [Member] | Loans and Finance Receivables [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Junior subordinated debt, Significant Unobservable Inputs	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]
Junior subordinated debt, Significant Unobservable Inputs	Third party appraisal	Third party appraisal
Third Party Appraisal [Member] | Other Assets [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Junior subordinated debt, Significant Unobservable Inputs	Third party appraisal	Third party appraisal
Discount Rate [Member] | Loans and Finance Receivables [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Junior subordinated debt, Significant Unobservable Inputs	Discount rate	Discount rate
Scheduled Cash Collections [Member] | Loans and Finance Receivables [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Junior subordinated debt, Significant Unobservable Inputs	Scheduled cash collections	Scheduled cash collections
Proceeds from Non-Real Estate Collateral [Member] | Loans and Finance Receivables [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Junior subordinated debt, Significant Unobservable Inputs	Proceeds from non-real estate collateral	Proceeds from non-real estate collateral