Property and Casualty Insurance Reserves - Allowance for Credit Losses from Policyholders (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Liability for Future Policy Benefits and Unpaid Claims and Claims Adjustment Expense [Abstract]
Receivables from Policyholders, Net of Allowance for Expected Credit Losses	$ 1,249.3	$ 1,249.3	$ 1,165.3	$ 1,117.1
Allowance for Expected Credit Losses
Balance at Beginning of Year	29.9	22.3
Provision for Expected Credit Losses	3.5	31.9
Write-offs of Uncollectible Receivables from Policyholders	(13.0)	(33.8)
Balance at End of Period	$ 20.4	$ 20.4