Schedule V Valuation and Qualifying Accounts (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Real Estate reserve (deducted from other long-term investments)
Valuation and Qualifying Accounts Disclosure
Balance at Beginning of Year		$ 0.3	$ 1.5	$ 4.2
Additions Charged to Costs and Expenses		0.0	0.0	0.6
Additions Charged to Other Accounts	[1]	0.0	0.0	0.0
Deductions	[2]	0.3	1.2	3.3
Balance at End of Year		0.0	0.3	1.5
Allowance for expected credit losses (deducted from accounts and premiums receivable)
Valuation and Qualifying Accounts Disclosure
Balance at Beginning of Year		8.4	9.9	6.4
Additions Charged to Costs and Expenses		34.2	5.3	5.6
Additions Charged to Other Accounts	[1]	0.1	0.1	0.0
Deductions	[2]	19.3	6.9	2.1
Balance at End of Year		38.8	8.4	$ 9.9
Allowance for expected credit losses (deducted from accounts and premiums receivable) | Accounting Standards Update 2016-13 | Cumulative Effect, Period of Adoption, Adjusted Balance
Valuation and Qualifying Accounts Disclosure
Balance at Beginning of Year	[3]	23.8
Balance at End of Year	[3]		23.8
Allowance for expected credit losses (deducted from reinsurance recoverable)
Valuation and Qualifying Accounts Disclosure
Additions Charged to Costs and Expenses		10.1
Additions Charged to Other Accounts	[1]	0.0
Deductions	[2]	0.2
Balance at End of Year		11.7
Allowance for expected credit losses (deducted from reinsurance recoverable) | Accounting Standards Update 2016-13 | Cumulative Effect, Period of Adoption, Adjusted Balance
Valuation and Qualifying Accounts Disclosure
Balance at Beginning of Year	[3]	$ 1.8
Balance at End of Year	[3]		$ 1.8

[1] Additions charged to other accounts are comprised of amounts related to fluctuations in the foreign currency exchange rate.

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

[3] ASC 326 "Financial Instruments - Credit Losses" was adopted resulting in a beginning balance adjustment of $13.5 million to Allowance for expected credit losses (deducted from accounts and premiums receivable) and the now separately reported $1.8 million beginning balance for Allowance for expected credit losses (deducted from reinsurance recoverable).