Allowance for Credit Losses - Reinsurance Recoverables (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Financing Receivable, Allowance for Credit Loss [Line Items]
Reinsurance recoverables, net	$ 3,046.5	$ 3,104.6
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Balance, January 1, 2020	$ 1.1
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
Current period change for estimated uncollectible reinsurance	$ 0.1
Write-offs of uncollectible reinsurance recoverables	0.0
Balance, September 30, 2020	$ 3.7
Percent rated A- or better	89.70%
Cumulative Effect, Period of Adoption, Adjustment
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Balance, January 1, 2020	$ 2.5