Investments - Allowance for Credit Loss (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Rollforward of the allowance for credit loss by major security type
Beginning balance	$ 7.8	$ 15.5
Additions for credit losses not previously recorded		1.0
Additional increases (decreases) for credit losses on securities with an allowance recorded in the previous period	(4.2)	3.0
Foreign currency translation adjustment	0.5	0.9
Ending balance	4.1	20.4
Corporate debt securities
Rollforward of the allowance for credit loss by major security type
Beginning balance	7.7	15.1
Additions for credit losses not previously recorded		1.0
Additional increases (decreases) for credit losses on securities with an allowance recorded in the previous period	(4.2)	3.0
Foreign currency translation adjustment	0.5	0.9
Ending balance	4.0	20.0
Commercial mortgage-backed securities
Rollforward of the allowance for credit loss by major security type
Beginning balance		0.3
Ending balance		0.3
Other debt obligations
Rollforward of the allowance for credit loss by major security type
Beginning balance	0.1	0.1
Ending balance	$ 0.1	$ 0.1