Fair Value - Summary of Changes in Fair Value Associated with Level 3 Assets (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	$ 113.8	$ 122.4	$ 110.1	$ 121.9
Calls/ Maturities/ Paydowns	(0.4)	1.6	3.3	2.1
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	(12.1)	(12.0)	(12.1)	(12.0)
Net Transfers In (Out)	0.0	0.0	0.0	0.0
Fair value, ending balance	101.3	112.0	101.3	112.0
Corporate 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 | Financials
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	49.2	67.4	49.2	67.4
Calls/ Maturities/ Paydowns	0.0	0.0	0.0	0.0
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	(12.5)	(11.4)	(12.5)	(11.4)
Net Transfers In (Out)	0.0	0.0	0.0	0.0
Fair value, ending balance	36.7	56.0	36.7	56.0
Nonredeemable preferred stocks | Industrials
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	14.8	16.4	14.8	16.4
Calls/ Maturities/ Paydowns	0.0	0.0	0.0	0.0
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	0.4	(0.6)	0.4	(0.6)
Net Transfers In (Out)	0.0	0.0	0.0	0.0
Fair value, ending balance	15.2	15.8	15.2	15.8
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	0.0	0.0	0.0
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	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	24.3	20.3	20.6	19.8
Calls/ Maturities/ Paydowns	(0.4)	1.6	3.3	2.1
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	0.0	0.0
Net Transfers In (Out)	0.0	0.0	0.0	0.0
Fair value, ending balance	$ 23.9	$ 21.9	$ 23.9	$ 21.9