SCHEDULE V VALUATION AND QUALIFYING ACCOUNTS (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Valuation And Qualifying Accounts Disclosure [Line Items]
Balance at beginning of period		$ 7.4	$ 8.1	$ 14.0
Charged to costs and expenses		19.6	9.5	10.1
Charged to other accounts	[1]			(6.4)
Deductions		(11.1)	(10.2)	(9.6)
Balance at end of period		15.9	7.4	8.1
Allowance for doubtful accounts [Member]
Valuation And Qualifying Accounts Disclosure [Line Items]
Balance at beginning of period		3.5	4.2	4.3
Charged to costs and expenses		16.9	9.5	9.5
Deductions		(11.1)	(10.2)	(9.6)
Balance at end of period		9.3	3.5	4.2
Allowance for uncollectible reinsurance recoverables [Member]
Valuation And Qualifying Accounts Disclosure [Line Items]
Balance at beginning of period		3.9	3.9	9.7
Charged to costs and expenses		2.7		0.6
Charged to other accounts	[1]			(6.4)
Balance at end of period		$ 6.6	$ 3.9	$ 3.9

[1] Amounts charged to other accounts include foreign exchange gains and losses. Additionally, in 2018 the Company sold its former Chaucer business; accordingly, the $6.4 million balance associated with this business was reduced as part of the sale.