Investments - Schedule of Investments' Gross Unrealized Loss and Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	$ 5.9	$ 8.9
Unrealized Losses, Less than 12 Months	(0.1)	(0.2)
Fair Value, 12 Months or More	22.9	19.9
Unrealized Losses, 12 Months or More	(1.8)	(2.0)
Fair Value	28.8	28.8
Unrealized Losses	(1.9)	(2.2)
U.S. Treasury securities, obligations of U.S. Government agencies and municipalities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	5.9	4.7
Unrealized Losses, Less than 12 Months	(0.1)	(0.1)
Fair Value, 12 Months or More	18.1	16.3
Unrealized Losses, 12 Months or More	(1.5)	(1.7)
Fair Value	24.0	21.0
Unrealized Losses	(1.6)	(1.8)
Corporate debt
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months		4.2
Unrealized Losses, Less than 12 Months		(0.1)
Fair Value, 12 Months or More	4.8	3.6
Unrealized Losses, 12 Months or More	(0.3)	(0.3)
Fair Value	4.8	7.8
Unrealized Losses	$ (0.3)	$ (0.4)