Allowance for Credit Losses - Premium Receivables (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Jan. 01, 2019	Jan. 01, 2018
Financing Receivable, Allowance for Credit Loss [Line Items]
Premiums receivable	$ 679.8	$ 790.2	$ 785.0	$ 718.2	$ 676.5	$ 727.5	$ 716.8	$ 664.9	$ 645.2	$ 594.0
Premium Receivable, Allowance for Credit Loss [Roll Forward]
Balance, January 1, 2020	$ 7.9
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
Current period change for estimated uncollectible premiums	$ 3.6
Write-offs of uncollectible premiums receivable	(2.4)
Foreign exchange adjustments	0.3
Balance, December 31, 2020	9.4
Cumulative Effect, Period of Adoption, Adjustment
Premium Receivable, Allowance for Credit Loss [Roll Forward]
Balance, January 1, 2020	$ 0.0