Investments - Aging of Gross Unrealized Losses (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Fair Value
Less than 12 Months	$ 207.2	$ 168.6
12 Months or More	214.5	236.2
Fair Value	421.7	404.8
Gross Unrealized Losses
Less than 12 Months	(1.2)	(1.4)
12 Months or More	(3.2)	(4.9)
Gross Unrealized Losses	(4.4)	(6.3)
Corporate debt securities
Fair Value
Less than 12 Months	141.5	89.0
12 Months or More	165.8	178.3
Fair Value	307.3	267.3
Gross Unrealized Losses
Less than 12 Months	(0.6)	(1.2)
12 Months or More	(2.9)	(4.0)
Gross Unrealized Losses	(3.5)	(5.2)
U.S. Government obligations
Fair Value
Less than 12 Months	64.5	79.6
12 Months or More	48.6	57.7
Fair Value	113.1	137.3
Gross Unrealized Losses
Less than 12 Months	(0.6)	(0.2)
12 Months or More	(0.3)	(0.9)
Gross Unrealized Losses	(0.9)	(1.1)
Asset-backed securities
Fair Value
Less than 12 Months	1.2	0.0
12 Months or More	0.1	0.2
Fair Value	1.3	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