Fair Value - Summary of Changes in Fair Value Associated with Level 3 Assets (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	$ 101.3	$ 112.0	$ 110.1	$ 121.9
Calls/ Maturities/ Paydowns	(1.4)	0.7	1.9	2.8
Purchases	0.0	0.0	0.0	0.0
Sales	0.0	(10.0)	0.0	(10.0)
Net Realized (Gain)/Loss on Sales	0.0	8.9	0.0	8.9
Change in Valuation	0.0	(7.0)	(12.1)	(19.0)
Net Transfers In (Out)	0.0	0.0	0.0	0.0
Fair value, ending balance	99.9	104.6	99.9	104.6
Corporate and other debt securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	3.0		3.0
Calls/ Maturities/ Paydowns	0.0		0.0
Purchases	0.0		0.0
Sales	0.0		0.0
Net Realized (Gain)/Loss on Sales	0.0		0.0
Change in Valuation	0.0		0.0
Net Transfers In (Out)	0.0		0.0
Fair value, ending balance	3.0		3.0
Nonredeemable preferred stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	51.9	71.8	64.0	83.8
Calls/ Maturities/ Paydowns	0.0	(8.1)	0.0	(8.1)
Purchases	0.0	0.0	0.0	0.0
Sales	0.0	0.0	0.0	0.0
Net Realized (Gain)/Loss on Sales	0.0	0.0	0.0	0.0
Change in Valuation	0.0	0.0	(12.1)	(12.0)
Net Transfers In (Out)	0.0	0.0	0.0	0.0
Fair value, ending balance	51.9	63.7	51.9	63.7
Common equities | Common Stocks, excluding Other Risk
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	22.5	18.3	22.5	18.3
Calls/ Maturities/ Paydowns	0.0	8.1	0.0	8.1
Purchases	0.0	0.0	0.0	0.0
Sales	0.0	(10.0)	0.0	(10.0)
Net Realized (Gain)/Loss on Sales	0.0	8.9	0.0	8.9
Change in Valuation	0.0	(7.0)	0.0	(7.0)
Net Transfers In (Out)	0.0	0.0	0.0	0.0
Fair value, ending balance	22.5	18.3	22.5	18.3
Common equities | Other risk investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	23.9	21.9	20.6	19.8
Calls/ Maturities/ Paydowns	(1.4)	0.7	1.9	2.8
Purchases	0.0	0.0	0.0	0.0
Sales	0.0	0.0	0.0	0.0
Net Realized (Gain)/Loss on Sales	0.0	0.0	0.0	0.0
Change in Valuation	0.0	0.0	0.0	0.0
Net Transfers In (Out)	0.0	0.0	0.0	0.0
Fair value, ending balance	$ 22.5	$ 22.6	$ 22.5	$ 22.6