Valuation and Qualifying Accounts (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Allowance for Credit Losses for Reinsurance Recoverable
Valuation And Qualifying Accounts Disclosure [Line Items]
Beginning Balance	$ 7.9	$ 0.0	$ 0.0
Charged to Costs and Expenses	0.0	4.6	0.0
Charged to Other Accounts	0.0	3.3	0.0
Deductions	4.7	0.0	0.0
Ending Balance	3.2	7.9	0.0
Allowance for Credit Losses for Premiums Receivable
Valuation And Qualifying Accounts Disclosure [Line Items]
Beginning Balance	2.2	0.7	0.4
Charged to Costs and Expenses	0.0	1.1	0.9
Charged to Other Accounts	0.0	0.4	0.0
Deductions	1.2	0.0	0.6
Ending Balance	$ 1.0	$ 2.2	$ 0.7