Investments - Schedule of Allowance for Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance	$ 0.5	$ 2.5	$ 0.2	$ 2.8
Securities for which allowance was not previously recorded	0.2	1.9	0.6	2.7
Securities sold during the period		(0.1)	(0.1)	(0.5)
Additional net increases (decreases) in existing allowance	0.3	0.1	0.3	(0.6)
Ending balance	1.0	4.4	1.0	4.4
Foreign Governments
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.0	1.0	0.2	0.7
Securities for which allowance was not previously recorded	0.0	0.0	0.0	0.1
Securities sold during the period		0.0	0.0	0.0
Additional net increases (decreases) in existing allowance	0.0	0.1	(0.2)	0.3
Ending balance	0.0	1.1	0.0	1.1
Obligations of states and political subdivisions
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.0	0.0	0.0	0.4
Securities for which allowance was not previously recorded	0.0	0.5	0.0	0.5
Securities sold during the period		0.0	0.0	0.0
Additional net increases (decreases) in existing allowance	0.0	0.0	0.0	(0.4)
Ending balance	0.0	0.5	0.0	0.5
Corporate bonds
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.5	1.4	0.0	1.6
Securities for which allowance was not previously recorded	0.2	1.4	0.6	2.1
Securities sold during the period		(0.1)	(0.1)	(0.5)
Additional net increases (decreases) in existing allowance	0.3	0.0	0.5	(0.5)
Ending balance	1.0	2.7	1.0	2.7
Asset-backed securities
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.0	0.1	0.0	0.1
Securities for which allowance was not previously recorded	0.0	0.0	0.0	0.0
Securities sold during the period		0.0	0.0	0.0
Additional net increases (decreases) in existing allowance	0.0	0.0	0.0	0.0
Ending balance	$ 0.0	$ 0.1	$ 0.0	$ 0.1