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, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	$ 115.4	$ 54.8	$ 127.7	$ 54.8
Calls/ Maturities/ Paydowns	(1.6)	8.4	1.1	8.5
Purchases	0.0	65.2	0.0	65.2
Sales	0.0	(5.0)	(15.0)	(5.0)
Net Realized (Gain)/Loss on Sales	0.0	(4.5)	(17.2)	(4.5)
Change in Valuation	(13.4)	56.0	3.8	55.9
Net Transfers In (Out)	0.0	0.0	0.0	0.0
Fair value, ending balance	100.4	174.9	100.4	174.9
Total equity securities | Financials | Nonredeemable preferred stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	61.4	10.0	76.4	10.0
Calls/ Maturities/ Paydowns	0.0	0.0	0.0	0.0
Purchases	0.0	60.2	0.0	60.2
Sales	0.0	0.0	(15.0)	0.0
Net Realized (Gain)/Loss on Sales	0.0	0.0	(17.2)	0.0
Change in Valuation	3.5	6.2	20.7	6.2
Net Transfers In (Out)	0.0	0.0	0.0	0.0
Fair value, ending balance	64.9	76.4	64.9	76.4
Total equity securities | Industrials | Nonredeemable preferred stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	33.9	16.6	34.4	16.7
Calls/ Maturities/ Paydowns	0.0	0.0	(0.5)	0.0
Purchases	0.0	5.0	0.0	5.0
Sales	0.0	(5.0)	0.0	(5.0)
Net Realized (Gain)/Loss on Sales	0.0	(4.5)	0.0	(4.5)
Change in Valuation	(16.9)	22.3	(16.9)	22.2
Net Transfers In (Out)	0.0	0.0	0.0	0.0
Fair value, ending balance	17.0	34.4	17.0	34.4
Total equity securities | Common Stocks, excluding Other Risk | Common equities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance		25.0		25.0
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		27.5		27.5
Net Transfers In (Out)		0.0		0.0
Fair value, ending balance		52.5		52.5
Total equity securities | Other risk investments | Common equities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	20.1	3.2	16.9	3.1
Calls/ Maturities/ Paydowns	(1.6)	8.4	1.6	8.5
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	$ 18.5	$ 11.6	$ 18.5	$ 11.6