Fair Value Measurements - Quantitative Information for Level 3 Inputs (Details) $ in Millions	Mar. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity Securities, FV-NI, Current	$ 243.6	$ 243.2
Fair Value, Equity Method Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments	96.4	95.9
Fair Value, Alternative Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments	100.5	96.3
Equity Securities, FV-NI	37.1	38.4
Fair Value, Equity Securities FV-NI, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments	0.0	0.0
Unfunded commitments		192.2
Mezzanine Debt
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value, Equity Method Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments	53.3	51.6
Fair Value, Alternative Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments	62.4	56.0
Senior Debt
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value, Equity Method Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments	40.7	42.0
Fair Value, Alternative Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments	6.0	6.0
Distressed Debt
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value, Equity Method Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments	0.0	0.0
Fair Value, Alternative Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments	13.0	13.0
Secondary Transactions
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value, Equity Method Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments	1.7	1.7
Fair Value, Alternative Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments	4.3	4.2
Hedge Funds
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value, Equity Method Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments	0.0	0.0
Fair Value, Alternative Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments	0.0	0.0
Leveraged Buyout
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value, Equity Method Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments	0.7	0.6
Fair Value, Alternative Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments	7.9	9.0
Equity Funds
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value, Equity Method Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments	0.0	0.0
Fair Value, Alternative Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments	6.7	7.9
Real Estate Funds
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value, Equity Method Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments	0.0	0.0
Other Funds
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value, Equity Method Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments	0.0	0.0
Fair Value, Alternative Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments	0.0	0.2
Alternative Energy Partnership Investments | Variable Interest Entity, Not Primary Beneficiary
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Outstanding commitments to fund equity method limited liability investments	0.0	0.0
Real Estate
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value, Alternative Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments	0.2	0.0
Fair Value, Measurements, Recurring
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Assets	7,486.3	7,183.0
Total Liabilities		0.4
Fair Value, Measurements, Recurring | Designated as Hedging Instrument
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative Liability		0.4
Fair Value, Measurements, Recurring | Limited Liability Companies and Limited Partnerships
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity Securities, FV-NI, Current	195.6	189.1
Fair Value, Measurements, Recurring | Other Industries | Preferred Stocks
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity Securities, FV-NI, Current	11.2	10.8
Fair Value, Measurements, Recurring | Other Industries | Common Stock
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity Securities, FV-NI, Current	0.6	1.2
Significant Unobservable Inputs (Level 3) | Limited Liability Companies and Limited Partnerships
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Unfunded commitments	196.9
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Assets	210.2	230.0
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 | Limited Liability Companies and Limited Partnerships
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity Securities, FV-NI, Current	0.0	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	1.6	1.6
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	0.4	0.5
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Assets	128.4	117.0
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 | Limited Liability Companies and Limited Partnerships
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 | 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	0.3
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Assets	6,952.1	6,646.9
Derivative Liability		0.4
Total Liabilities		0.4
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Limited Liability Companies and Limited Partnerships
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity Securities, FV-NI, Current	0.0	0.0
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	9.6	9.2
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	0.0	0.4
Fair Value Measured at Net Asset Value Per Share
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Assets	439.6	430.7
Equity Method Investments, Fair Value Disclosure	218.7	217.0
Alternative Investment	195.6	189.1
Equity Securities, FV-NI	8.3	8.3
Fair Value Measured at Net Asset Value Per Share | Mezzanine Debt
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity Method Investments, Fair Value Disclosure	117.3	114.3
Alternative Investment	111.4	106.0
Fair Value Measured at Net Asset Value Per Share | Senior Debt
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity Method Investments, Fair Value Disclosure	21.8	21.6
Alternative Investment	21.9	21.9
Fair Value Measured at Net Asset Value Per Share | Distressed Debt
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity Method Investments, Fair Value Disclosure	8.3	9.4
Alternative Investment	12.7	12.5
Fair Value Measured at Net Asset Value Per Share | Secondary Transactions
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity Method Investments, Fair Value Disclosure	9.1	9.3
Alternative Investment	3.1	3.5
Fair Value Measured at Net Asset Value Per Share | Hedge Funds
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity Method Investments, Fair Value Disclosure	0.1	0.5
Alternative Investment	17.7	18.1
Fair Value Measured at Net Asset Value Per Share | Leveraged Buyout
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity Method Investments, Fair Value Disclosure	9.2	8.9
Alternative Investment	22.3	21.6
Fair Value Measured at Net Asset Value Per Share | Equity Funds
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity Method Investments, Fair Value Disclosure	1.2	1.2
Alternative Investment	6.3	5.4
Fair Value Measured at Net Asset Value Per Share | Real Estate Funds
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity Method Investments, Fair Value Disclosure	43.3	43.3
Fair Value Measured at Net Asset Value Per Share | Other Funds
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity Method Investments, Fair Value Disclosure	8.4	8.5
Alternative Investment	0.0	0.1
Fair Value Measured at Net Asset Value Per Share | Alternative Energy Partnership Investments
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity Method Investments, Fair Value Disclosure	17.0	16.3
Fair Value Measured at Net Asset Value Per Share | Real Estate
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Alternative Investment	0.2	0.0
Measured at Net Asset Value | Fair Value, Measurements, Recurring
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Assets	195.6	189.1
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 | Limited Liability Companies and Limited Partnerships
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity Securities, FV-NI, Current	195.6	189.1
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	0.046	0.046
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	0.367	0.367
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	0.128	0.109
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	0.115	0.088
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	0.233	0.225
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	0.152	0.151
Private Placement | Investment-grade | Market Yield | Significant Unobservable Inputs (Level 3)
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	$ 64.1	$ 56.5
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	0.042	0.046
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	0.140	0.145
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	0.089	0.092
Senior Debt | Non-investment-grade | Market Yield | Significant Unobservable Inputs (Level 3)
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	$ 48.6	$ 72.9
Junior Debt | Non-investment-grade | Market Yield | Significant Unobservable Inputs (Level 3)
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	36.8	42.1
Other | Market Yield | Significant Unobservable Inputs (Level 3)
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	58.7	56.4
Investments in Fixed Maturities | Market Yield | Significant Unobservable Inputs (Level 3)
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Fair Value	$ 208.2	$ 227.9