Investments - Allowance for Credit Loss (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($)
Rollforward of the allowance for credit loss by major security type
Additions for credit losses not previously recorded	$ 16.2
Reductions for securities sold during the period	(7.0)
Additional increases (decreases) for credit losses on securities with an allowance recorded in the previous period	0.2
Write-offs charged against allowance	(2.6)
Foreign currency translation adjustment	0.6
Ending balance	7.4
Corporate debt securities
Rollforward of the allowance for credit loss by major security type
Additions for credit losses not previously recorded	13.2
Reductions for securities sold during the period	(7.0)
Additional increases (decreases) for credit losses on securities with an allowance recorded in the previous period	(5.9)
Foreign currency translation adjustment	0.6
Ending balance	0.9
Commercial mortgage-backed securities
Rollforward of the allowance for credit loss by major security type
Additions for credit losses not previously recorded	2.9
Additional increases (decreases) for credit losses on securities with an allowance recorded in the previous period	4.0
Write-offs charged against allowance	(2.6)
Ending balance	4.3
Collateralized debt obligations
Rollforward of the allowance for credit loss by major security type
Additions for credit losses not previously recorded	0.1
Additional increases (decreases) for credit losses on securities with an allowance recorded in the previous period	2.1
Ending balance	$ 2.2