Investments - Allowance for Credit Loss (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Rollforward of the allowance for credit loss by major security type
Beginning balance	$ 8.7	$ 7.2	$ 7.4
Additions for credit losses not previously recorded	8.0	4.6	15.9	$ 16.3
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		1.9	2.8	4.4
Write-offs charged against allowance		(1.6)	(9.4)	(1.6)
Foreign currency translation adjustment	(0.4)		(0.4)
Ending balance	16.3	12.1	16.3	12.1
Accrued interest written off to net investment income			0.2
Corporate debt securities
Rollforward of the allowance for credit loss by major security type
Beginning balance	8.5	2.8	0.9
Additions for credit losses not previously recorded	7.8	3.4	15.4	13.2
Reductions for securities sold during the period				(7.0)
Foreign currency translation adjustment	(0.4)		(0.4)
Ending balance	15.9	6.2	15.9	6.2
Accrued interest written off to net investment income			0.2
Commercial mortgage-backed securities
Rollforward of the allowance for credit loss by major security type
Beginning balance	0.2	3.4	4.3
Additions for credit losses not previously recorded	0.1	1.2	0.4	2.9
Additional increases (decreases) for credit losses on securities with an allowance recorded in the previous period		1.0	2.4	2.7
Write-offs charged against allowance		(1.6)	(6.8)	(1.6)
Ending balance	0.3	4.0	0.3	4.0
Collateralized debt obligations
Rollforward of the allowance for credit loss by major security type
Beginning balance		1.0	2.2
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		0.9	0.4	1.8
Write-offs charged against allowance			(2.6)
Ending balance		$ 1.9		1.9
Other debt obligations
Rollforward of the allowance for credit loss by major security type
Additions for credit losses not previously recorded	0.1		0.1	0.1
Additional increases (decreases) for credit losses on securities with an allowance recorded in the previous period				$ (0.1)
Ending balance	$ 0.1		$ 0.1