Investments - Aging of Gross Unrealized Losses (Details) - USD ($) $ in Millions	Jun. 30, 2023	Dec. 31, 2022
Fair Value
Less than 12 Months	$ 283.8	$ 116.0
12 Months or More	352.8	513.2
Fair Value	636.6	629.2
Gross Unrealized Losses
Less than 12 Months	(3.3)	(2.3)
12 Months or More	(13.1)	(21.0)
Gross Unrealized Losses	(16.4)	(23.3)
Corporate debt securities
Fair Value
Less than 12 Months	184.5	83.6
12 Months or More	299.6	428.1
Fair Value	484.1	511.7
Gross Unrealized Losses
Less than 12 Months	(2.4)	(2.1)
12 Months or More	(10.7)	(17.5)
Gross Unrealized Losses	(13.1)	(19.6)
U.S. Government obligations
Fair Value
Less than 12 Months	97.8	29.6
12 Months or More	53.2	85.1
Fair Value	151.0	114.7
Gross Unrealized Losses
Less than 12 Months	(0.9)	(0.2)
12 Months or More	(2.4)	(3.5)
Gross Unrealized Losses	(3.3)	(3.7)
Asset-backed securities
Fair Value
Less than 12 Months	1.5	2.8
12 Months or More	0.0	0.0
Fair Value	1.5	2.8
Gross Unrealized Losses
Less than 12 Months	0.0	0.0
12 Months or More	0.0	0.0
Gross Unrealized Losses	$ 0.0	$ 0.0