Allowance for Credit Losses - Premiums Receivable (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Dec. 31, 2019	[1]
Financing Receivable, Allowance for Credit Loss [Line Items]
Premiums receivable	$ 797.0	$ 688.2
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	$ 1.0
Write-offs of uncollectible premiums receivable	0.0
Foreign exchange adjustments	0.2
Balance, September 30, 2020	9.1
Cumulative Effect, Period of Adoption, Adjustment
Premium Receivable, Allowance for Credit Loss [Roll Forward]
Balance, January 1, 2020	$ 0.0

[1] Derived from audited consolidated financial statements.