Schedule V Valuation and Qualifying Accounts (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Allowance for expected credit losses (deducted from accounts and premiums receivable)
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Year		$ 32.5	$ 34.2	$ 38.8
Additions Charged to Costs and Expenses		11.3	10.8	18.2
Deductions	[1]	14.3	12.5	22.8
Balance at End of Year		29.5	32.5	34.2
Allowance for expected credit losses (deducted from reinsurance recoverable)
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Year		1.7	2.3	11.7
Additions Charged to Costs and Expenses		0.1	0.0	0.7
Deductions	[1]	0.1	0.6	10.1
Balance at End of Year		$ 1.7	$ 1.7	$ 2.3

[1] Deductions include amounts deemed to reduce exposure of expected losses on premium and accounts receivables and reinsurance recoverable, amounts deemed uncollectible, and amounts related to fluctuations in the foreign currency exchange rate.