Investments - Aging of Gross Unrealized Losses (Details) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Fair Value
Less than 12 Months	$ 213.7	$ 168.6
12 Months or More	167.9	236.2
Fair Value	381.6	404.8
Gross Unrealized Losses
Less than 12 Months	(1.1)	(1.4)
12 Months or More	(2.5)	(4.9)
Gross Unrealized Losses	(3.6)	(6.3)
Corporate debt securities
Fair Value
Less than 12 Months	163.0	89.0
12 Months or More	145.9	178.3
Fair Value	308.9	267.3
Gross Unrealized Losses
Less than 12 Months	(0.9)	(1.2)
12 Months or More	(2.3)	(4.0)
Gross Unrealized Losses	(3.2)	(5.2)
U.S. Government obligations
Fair Value
Less than 12 Months	40.8	79.6
12 Months or More	22.0	57.7
Fair Value	62.8	137.3
Gross Unrealized Losses
Less than 12 Months	(0.2)	(0.2)
12 Months or More	(0.2)	(0.9)
Gross Unrealized Losses	(0.4)	(1.1)
Asset-backed securities
Fair Value
Less than 12 Months	9.9	0.0
12 Months or More	0.0	0.2
Fair Value	9.9	0.2
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