Fair Value Measurements - Quantitative Information for Level 3 Inputs (Details) $ in Millions	Jun. 30, 2022 USD ($)	Dec. 31, 2021 USD ($)
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity Securities at Fair Value (Cost: 2022 - $363.1; 2021 - $618.7)	$ 395.0	$ 830.6
Fair Value, Measurements, Recurring
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total	7,657.3	8,863.9
Total Liabilities	3.6
Fair Value, Measurements, Recurring | Designated as Hedging Instrument
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative Liability	3.6
Fair Value, Measurements, Recurring | Other Industries | Preferred Stocks
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity Securities at Fair Value (Cost: 2022 - $363.1; 2021 - $618.7)	15.7	17.6
Fair Value, Measurements, Recurring | Other Industries | Common Stock
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity Securities at Fair Value (Cost: 2022 - $363.1; 2021 - $618.7)	0.6	2.9
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total	191.3	251.4
Total Liabilities	0.0
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Designated as Hedging Instrument
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative Liability	0.0
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Other Industries | Preferred Stocks
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity Securities at Fair Value (Cost: 2022 - $363.1; 2021 - $618.7)	1.7	1.5
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Other Industries | Common Stock
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity Securities at Fair Value (Cost: 2022 - $363.1; 2021 - $618.7)	0.0	0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total	198.9	586.6
Total Liabilities	0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring | Designated as Hedging Instrument
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative Liability	0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring | Other Industries | Preferred Stocks
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity Securities at Fair Value (Cost: 2022 - $363.1; 2021 - $618.7)	0.0	0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring | Other Industries | Common Stock
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity Securities at Fair Value (Cost: 2022 - $363.1; 2021 - $618.7)	0.6	2.9
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total	6,995.2	7,700.9
Total Liabilities	3.6
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Designated as Hedging Instrument
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative Liability	3.6
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Other Industries | Preferred Stocks
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity Securities at Fair Value (Cost: 2022 - $363.1; 2021 - $618.7)	14.0	16.1
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Other Industries | Common Stock
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity Securities at Fair Value (Cost: 2022 - $363.1; 2021 - $618.7)	0.0	0.0
Measured at Net Asset Value | Fair Value, Measurements, Recurring
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total	271.9	325.0
Total Liabilities	0.0
Measured at Net Asset Value | Fair Value, Measurements, Recurring | Designated as Hedging Instrument
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative Liability	0.0
Measured at Net Asset Value | Fair Value, Measurements, Recurring | Other Industries | Preferred Stocks
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity Securities at Fair Value (Cost: 2022 - $363.1; 2021 - $618.7)	0.0	0.0
Measured at Net Asset Value | Fair Value, Measurements, Recurring | Other Industries | Common Stock
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity Securities at Fair Value (Cost: 2022 - $363.1; 2021 - $618.7)	$ 0.0	$ 0.0
Senior Debt | Measurement Input, Discount Rate | Non-investment-grade | Market Yield | Minimum | Significant Unobservable Inputs (Level 3)
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.053	0.051
Senior Debt | Measurement Input, Discount Rate | Non-investment-grade | Market Yield | Maximum | Significant Unobservable Inputs (Level 3)
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.256	0.202
Senior Debt | Measurement Input, Discount Rate | Non-investment-grade | Market Yield | Weighted-average Yield | Significant Unobservable Inputs (Level 3)
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.123	0.085
Junior Debt | Measurement Input, Discount Rate | Non-investment-grade | Market Yield | Minimum | Significant Unobservable Inputs (Level 3)
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.060	0.060
Junior Debt | Measurement Input, Discount Rate | Non-investment-grade | Market Yield | Maximum | Significant Unobservable Inputs (Level 3)
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.250	0.275
Junior Debt | Measurement Input, Discount Rate | Non-investment-grade | Market Yield | Weighted-average Yield | Significant Unobservable Inputs (Level 3)
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.172	0.150
Private Placement | Investment-grade | Market Yield | Significant Unobservable Inputs (Level 3)
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	$ 60.9	$ 87.9
Private Placement | Measurement Input, Discount Rate | Investment-grade | Market Yield | Minimum | Significant Unobservable Inputs (Level 3)
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Alternative investment, measurement input	0.046	0.023
Private Placement | Measurement Input, Discount Rate | Investment-grade | Market Yield | Maximum | Significant Unobservable Inputs (Level 3)
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Alternative investment, measurement input	0.106	0.103
Private Placement | Measurement Input, Discount Rate | Investment-grade | Market Yield | Weighted-average Yield | Significant Unobservable Inputs (Level 3)
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Alternative investment, measurement input	0.069	0.054
Senior Debt | Non-investment-grade | Market Yield | Significant Unobservable Inputs (Level 3)
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	$ 26.7	$ 76.1
Junior Debt | Non-investment-grade | Market Yield | Significant Unobservable Inputs (Level 3)
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	43.8	53.9
Other | Market Yield | Significant Unobservable Inputs (Level 3)
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	58.2	32.0
Investments in Fixed Maturities | Market Yield | Significant Unobservable Inputs (Level 3)
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	$ 189.6	$ 249.9