Fair Value Measurements - Quantitative Information for Level 3 Inputs (Details) $ in Millions	Sep. 30, 2022 USD ($)	Dec. 31, 2021 USD ($)
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity Securities, FV-NI, Current	$ 322.7	$ 830.6
Fair Value, Measurements, Recurring
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total	7,044.4	8,863.9
Total Liabilities	0.3
Fair Value, Measurements, Recurring | Designated as Hedging Instrument
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative Liability	0.3
Fair Value, Measurements, Recurring | Other Industries | Preferred Stocks
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity Securities, FV-NI, Current	11.7	17.6
Fair Value, Measurements, Recurring | Other Industries | Common Stock
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity Securities, FV-NI, Current	1.1	2.9
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total	233.2	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, FV-NI, Current	2.0	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, FV-NI, Current	0.5	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	183.8	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, FV-NI, Current	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, FV-NI, Current	0.2	2.9
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total	6,418.9	7,700.9
Total Liabilities	0.3
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Designated as Hedging Instrument
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative Liability	0.3
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Other Industries | Preferred Stocks
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity Securities, FV-NI, Current	9.7	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, FV-NI, Current	0.4	0.0
Measured at Net Asset Value | Fair Value, Measurements, Recurring
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total	208.5	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, FV-NI, Current	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, FV-NI, Current	$ 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	0.062	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	0.345	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	0.103	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	0.091	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	0.279	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	0.177	0.150
Private Placement | Investment-grade | Market Yield | Significant Unobservable Inputs (Level 3)
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	$ 63.4	$ 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	0.129	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	0.086	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	$ 67.6	$ 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	48.0	53.9
Other | Market Yield | Significant Unobservable Inputs (Level 3)
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	51.7	32.0
Investments in Fixed Maturities | Market Yield | Significant Unobservable Inputs (Level 3)
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	$ 230.7	$ 249.9