Fair Value Measurements - Quantitative Information for Level 3 Inputs (Details) $ in Millions	Mar. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value, Equity Method Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments	$ 84.5	$ 85.3
Equity Securities, FV-NI	24.7	32.6
Fair Value, Equity Securities FV-NI, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments	0.0	0.0
Unfunded commitments		195.7
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	42.6	43.1
Fair Value, Alternative Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments	66.3	67.0
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	39.5	39.9
Fair Value, Alternative Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments	10.8	10.6
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	1.6	1.7
Fair Value, Alternative Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments	3.1	3.1
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	0.6	0.6
Fair Value, Alternative Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments	9.5	10.0
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	6.5	6.5
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	0.2	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	0.2
Fair Value, Measurements, Recurring
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Investments, Fair Value Disclosure	7,002.9	7,122.4
Derivative Asset		14.7
Cash Flow Hedge Derivative Instrument Assets at Fair Value	1.2
Fair Value, Measurements, Recurring | Partnership Interest [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity Securities, FV-NI, Current	194.1	191.4
Fair Value, Measurements, Recurring | All Industries Other than Finance, Insurance and Real Estate [Member] | Preferred Stocks
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity Securities, FV-NI, Current	10.0	9.9
Fair Value, Measurements, Recurring | All Industries Other than Finance, Insurance and Real Estate [Member] | Common Stock [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity Securities, FV-NI, Current	0.8	0.6
Fair Value, Inputs, Level 3 [Member] | Partnership Interest [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Unfunded commitments	193.9
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Investments, Fair Value Disclosure	191.9	192.3
Derivative Asset		0.0
Cash Flow Hedge Derivative Instrument Assets at Fair Value	0.0
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring | Partnership Interest [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity Securities, FV-NI, Current	0.0	0.0
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring | All Industries Other than Finance, Insurance and Real Estate [Member] | Preferred Stocks
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity Securities, FV-NI, Current	2.2	2.4
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring | All Industries Other than Finance, Insurance and Real Estate [Member] | Common Stock [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity Securities, FV-NI, Current	0.5	0.4
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Investments, Fair Value Disclosure	95.0	107.3
Derivative Asset		0.0
Cash Flow Hedge Derivative Instrument Assets at Fair Value	0.0
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring | Partnership Interest [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity Securities, FV-NI, Current	0.0	0.0
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring | All Industries Other than Finance, Insurance and Real Estate [Member] | Preferred Stocks
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity Securities, FV-NI, Current	0.0	0.0
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring | All Industries Other than Finance, Insurance and Real Estate [Member] | Common Stock [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity Securities, FV-NI, Current	0.3	0.2
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Investments, Fair Value Disclosure	6,521.9	6,631.4
Derivative Asset		14.7
Cash Flow Hedge Derivative Instrument Assets at Fair Value	1.2
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring | Partnership Interest [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity Securities, FV-NI, Current	0.0	0.0
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring | All Industries Other than Finance, Insurance and Real Estate [Member] | Preferred Stocks
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity Securities, FV-NI, Current	7.8	7.5
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring | All Industries Other than Finance, Insurance and Real Estate [Member] | Common Stock [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity Securities, FV-NI, Current	0.0	0.0
Fair Value Measured at Net Asset Value Per Share
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Investments, Fair Value Disclosure	413.9	417.9
Equity Method Investments, Fair Value Disclosure	215.2	221.7
Alternative Investment	194.1	191.4
Fair Value, Alternative Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments	109.4	110.4
Equity Securities, FV-NI	4.6	4.8
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	123.2	125.4
Alternative Investment	127.7	124.0
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	19.2	19.0
Alternative Investment	24.7	24.8
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	6.1	7.9
Alternative Investment	12.9	12.4
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	6.8	7.9
Alternative Investment	2.7	2.8
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	0.1
Alternative Investment	0.0	1.9
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	8.5	8.6
Alternative Investment	19.7	19.0
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	0.8	1.2
Alternative Investment	6.3	6.4
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	42.0	41.9
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	8.5	9.7
Fair Value Measured at Net Asset Value Per Share | Real Estate
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Alternative Investment	0.1	0.1
Fair Value Measured at Net Asset Value Per Share | Fair Value, Measurements, Recurring
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash Flow Hedge Derivative Instrument Assets at Fair Value	0.0
Fair Value, Inputs, Net Asset Value [Member] | Fair Value, Measurements, Recurring
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Investments, Fair Value Disclosure	194.1	191.4
Derivative Asset		0.0
Fair Value, Inputs, Net Asset Value [Member] | Fair Value, Measurements, Recurring | Partnership Interest [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity Securities, FV-NI, Current	194.1	191.4
Fair Value, Inputs, Net Asset Value [Member] | Fair Value, Measurements, Recurring | All Industries Other than Finance, Insurance and Real Estate [Member] | Preferred Stocks
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity Securities, FV-NI, Current	0.0	0.0
Fair Value, Inputs, Net Asset Value [Member] | Fair Value, Measurements, Recurring | All Industries Other than Finance, Insurance and Real Estate [Member] | Common Stock [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity Securities, FV-NI, Current	$ 0.0	$ 0.0
Senior Debt Obligations [Member] | Measurement Input, Discount Rate | Non-investment-grade | Market Yield | Minimum | Fair Value, Inputs, Level 3 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.070	0.092
Senior Debt Obligations [Member] | Measurement Input, Discount Rate | Non-investment-grade | Market Yield | Maximum | Fair Value, Inputs, Level 3 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.480	0.367
Senior Debt Obligations [Member] | Measurement Input, Discount Rate | Non-investment-grade | Market Yield | Weighted Average [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.130	0.135
Junior Subordinated Debt [Member] | Measurement Input, Discount Rate | Non-investment-grade | Market Yield | Minimum | Fair Value, Inputs, Level 3 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.110	0.118
Junior Subordinated Debt [Member] | Measurement Input, Discount Rate | Non-investment-grade | Market Yield | Maximum | Fair Value, Inputs, Level 3 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.255	0.225
Junior Subordinated Debt [Member] | Measurement Input, Discount Rate | Non-investment-grade | Market Yield | Weighted Average [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt instrument, measurement input	0.132	0.138
Private Placement | External Credit Rating, Investment Grade [Member] | Market Yield | Fair Value, Inputs, Level 3 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	$ 55.3	$ 60.0
Private Placement | Measurement Input, Discount Rate | External Credit Rating, Investment Grade [Member] | Market Yield | Minimum | Fair Value, Inputs, Level 3 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Alternative investment, measurement input	0.018	0.042
Private Placement | Measurement Input, Discount Rate | External Credit Rating, Investment Grade [Member] | Market Yield | Maximum | Fair Value, Inputs, Level 3 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Alternative investment, measurement input	0.135	0.158
Private Placement | Measurement Input, Discount Rate | External Credit Rating, Investment Grade [Member] | Market Yield | Weighted Average [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Alternative investment, measurement input	0.079	0.087
Senior Debt Obligations [Member] | Non-investment-grade | Market Yield | Fair Value, Inputs, Level 3 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	$ 31.7	$ 32.6
Junior Subordinated Debt [Member] | Non-investment-grade | Market Yield | Fair Value, Inputs, Level 3 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	35.8	32.5
Other Debt Obligations [Member] | Market Yield | Fair Value, Inputs, Level 3 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	66.4	64.4
Investments in Fixed Maturities | Market Yield | Fair Value, Inputs, Level 3 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	$ 189.2	$ 189.5