Investments - Aging of Gross Unrealized Losses (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Fair Value
Less than 12 Months	$ 321.3	$ 116.0
12 Months or More	279.5	513.2
Fair Value	600.8	629.2
Gross Unrealized Losses
Less than 12 Months	(3.2)	(2.3)
12 Months or More	(8.8)	(21.0)
Gross Unrealized Losses	(12.0)	(23.3)
Corporate debt securities
Fair Value
Less than 12 Months	204.5	83.6
12 Months or More	216.7	428.1
Fair Value	421.2	511.7
Gross Unrealized Losses
Less than 12 Months	(2.2)	(2.1)
12 Months or More	(6.9)	(17.5)
Gross Unrealized Losses	(9.1)	(19.6)
U.S. Government obligations
Fair Value
Less than 12 Months	116.8	29.6
12 Months or More	62.4	85.1
Fair Value	179.2	114.7
Gross Unrealized Losses
Less than 12 Months	(1.0)	(0.2)
12 Months or More	(1.9)	(3.5)
Gross Unrealized Losses	(2.9)	(3.7)
Asset-backed securities
Fair Value
Less than 12 Months	0.0	2.8
12 Months or More	0.4	0.0
Fair Value	0.4	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