Investments - Allowance for Credit Loss (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2020
Allowance for Credit Loss
Beginning balance	$ 7.6
Additions for credit losses on securities for which an allowance was not previously recorded	4.1	$ 11.7
Reductions for securities sold during the period	(7.0)	(7.0)
Additional increases (decreases) for credit losses on securities with an allowance recorded in the previous period	2.5	2.5
Ending balance	7.2	7.2
Corporate debt securities
Allowance for Credit Loss
Beginning balance	7.0
Additions for credit losses on securities for which an allowance was not previously recorded	2.8	9.8
Reductions for securities sold during the period	(7.0)	(7.0)
Ending balance	2.8	2.8
Commercial mortgage-backed securities
Allowance for Credit Loss
Beginning balance	0.4
Additions for credit losses on securities for which an allowance was not previously recorded	1.3	1.7
Additional increases (decreases) for credit losses on securities with an allowance recorded in the previous period	1.7	1.7
Ending balance	3.4	3.4
Collateralized debt obligations
Allowance for Credit Loss
Beginning balance	0.1
Additions for credit losses on securities for which an allowance was not previously recorded		0.1
Additional increases (decreases) for credit losses on securities with an allowance recorded in the previous period	0.9	0.9
Ending balance	1.0	1.0
Other debt obligations
Allowance for Credit Loss
Beginning balance	0.1
Additions for credit losses on securities for which an allowance was not previously recorded		0.1
Additional increases (decreases) for credit losses on securities with an allowance recorded in the previous period	$ (0.1)	$ (0.1)