Investments - Schedule of Reconciliation of Beginning and Ending Balances for Investments Categorized as Level 3 (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2024	Dec. 31, 2023
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 56.8	$ 56.2
Transfers into Level 3	0.7	5.6
Transfers out of Level 3	(10.1)	(13.2)
Included in net income	29.5	(0.5)
Included in other comprehensive income	0.0	0.8
Purchases	98.9	10.6
Sales	0.0	(1.6)
Settlements	(19.8)	(1.1)
Ending balance	156.0	56.8
Amount of total gains or losses for the year included in net income attributable to the change in unrealized gains or losses relating to assets still held at end of period	29.3	(0.8)
Fixed maturities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	50.4	40.7
Transfers into Level 3	0.7	5.6
Transfers out of Level 3	(10.1)	(5.6)
Included in net income	1.2	(0.1)
Included in other comprehensive income	0.0	0.8
Purchases	1.9	10.6
Sales	0.0	(0.5)
Settlements	(7.7)	(1.1)
Ending balance	36.4	50.4
Amount of total gains or losses for the year included in net income attributable to the change in unrealized gains or losses relating to assets still held at end of period	0.0	0.0
Equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	6.4	15.5
Transfers into Level 3	0.0	0.0
Transfers out of Level 3	0.0	(7.6)
Included in net income	28.3	(0.4)
Included in other comprehensive income	0.0	0.0
Purchases	47.2	0.0
Sales	0.0	(1.1)
Settlements	0.0	0.0
Ending balance	81.9	6.4
Amount of total gains or losses for the year included in net income attributable to the change in unrealized gains or losses relating to assets still held at end of period	29.3	(0.8)
Other Investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	0.0
Transfers into Level 3	0.0
Transfers out of Level 3	0.0
Included in net income	0.0
Included in other comprehensive income	0.0
Purchases	49.8
Sales	0.0
Settlements	(12.1)
Ending balance	37.7	$ 0.0
Amount of total gains or losses for the year included in net income attributable to the change in unrealized gains or losses relating to assets still held at end of period	$ 0.0