Fair Value Measurements - Valuation of Assets Measured at Fair Value (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)	$ 6,881.9	$ 6,894.8
Equity Securities at Fair Value (Cost: 2023 - $209.3; 2022 - $247.6)	$ 225.8	243.2	[1]
Derivative Asset, Statement of Financial Position [Extensible Enumeration]	Other Investments
Derivative Instruments Not Designated as Hedges		1.7
Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Convertible Securities at Fair Value		43.3
Derivative Instruments Not Designated as Hedges	$ 14.7
Total	7,122.4	7,183.0
Liabilities, Fair Value Disclosure		0.4
Fair Value, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Convertible Securities at Fair Value		0.0
Derivative Instruments Not Designated as Hedges	0.0	0.0
Total	107.3	117.0
Liabilities, Fair Value Disclosure		0.0
Fair Value, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Convertible Securities at Fair Value		43.3
Derivative Instruments Not Designated as Hedges	14.7	1.7
Total	6,631.4	6,646.9
Liabilities, Fair Value Disclosure		0.4
Fair Value, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Convertible Securities at Fair Value		0.0
Derivative Instruments Not Designated as Hedges	0.0	0.0
Total	192.3	230.0
Liabilities, Fair Value Disclosure		0.0
Investments in Fixed Maturities | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)	6,881.9	6,894.8
Investments in Fixed Maturities | Fair Value, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)	98.8	103.6
Investments in Fixed Maturities | Fair Value, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)	6,593.6	6,563.3
Investments in Fixed Maturities | Fair Value, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)	189.5	227.9
United States Government and Government Agencies and Authorities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)	511.5	528.0
United States Government and Government Agencies and Authorities | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)	511.5	528.0
United States Government and Government Agencies and Authorities | Fair Value, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)	98.8	103.6
United States Government and Government Agencies and Authorities | Fair Value, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)	412.7	424.4
United States Government and Government Agencies and Authorities | Fair Value, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)	0.0	0.0
States and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)	1,401.9	1,568.9
States and Political Subdivisions | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)	1,401.9	1,568.9
States and Political Subdivisions | Fair Value, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)	0.0	0.0
States and Political Subdivisions | Fair Value, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)	1,401.8	1,568.9
States and Political Subdivisions | Fair Value, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)	0.1	0.0
Debt Security, Government, Non-US [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)	3.8	4.1
Debt Security, Government, Non-US [Member] | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)	3.8	4.1
Debt Security, Government, Non-US [Member] | Fair Value, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)	0.0	0.0
Debt Security, Government, Non-US [Member] | Fair Value, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)	3.8	4.1
Debt Security, Government, Non-US [Member] | Fair Value, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)	0.0	0.0
Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)	3,690.8	3,539.4
Corporate Debt Securities [Member] | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)	3,690.8	3,539.4
Corporate Debt Securities [Member] | Fair Value, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)	0.0	0.0
Corporate Debt Securities [Member] | Fair Value, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)	3,513.7	3,323.4
Corporate Debt Securities [Member] | Fair Value, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)	177.1	216.0
Redeemable Preferred Stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)	8.3	8.0
Redeemable Preferred Stocks | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)	8.3
Redeemable Preferred Stocks | Fair Value, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)		0.0
Redeemable Preferred Stocks | Fair Value, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)		1.2
Redeemable Preferred Stocks | Fair Value, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)		6.8
Collateralized Loan Obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)	949.8	953.9
Collateralized Loan Obligations | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)	949.8	953.9
Collateralized Loan Obligations | Fair Value, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)	0.0	0.0
Collateralized Loan Obligations | Fair Value, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)	949.8	953.9
Collateralized Loan Obligations | Fair Value, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)	0.0	0.0
Other Mortgage- and Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)	315.8	292.5
Other Mortgage- and Asset-backed | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)	315.8	292.5
Other Mortgage- and Asset-backed | Fair Value, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)	0.0	0.0
Other Mortgage- and Asset-backed | Fair Value, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)	310.6	287.4
Other Mortgage- and Asset-backed | Fair Value, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)	5.2	5.1
Dividends on Equity Securities Excluding Alternative Investments | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2023 - $209.3; 2022 - $247.6)	225.8	243.2
Dividends on Equity Securities Excluding Alternative Investments | Fair Value, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2023 - $209.3; 2022 - $247.6)	8.5	13.4
Dividends on Equity Securities Excluding Alternative Investments | Fair Value, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2023 - $209.3; 2022 - $247.6)	23.1	38.6
Dividends on Equity Securities Excluding Alternative Investments | Fair Value, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2023 - $209.3; 2022 - $247.6)	2.8	2.1
Preferred Stocks | Finance, Insurance and Real Estate | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2023 - $209.3; 2022 - $247.6)	15.6	29.0
Preferred Stocks | Finance, Insurance and Real Estate | Fair Value, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2023 - $209.3; 2022 - $247.6)	0.0	0.0
Preferred Stocks | Finance, Insurance and Real Estate | Fair Value, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2023 - $209.3; 2022 - $247.6)	15.6	29.0
Preferred Stocks | Finance, Insurance and Real Estate | Fair Value, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2023 - $209.3; 2022 - $247.6)	0.0	0.0
Preferred Stocks | Other Industries | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2023 - $209.3; 2022 - $247.6)	9.9	10.8
Preferred Stocks | Other Industries | Fair Value, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2023 - $209.3; 2022 - $247.6)	0.0	0.0
Preferred Stocks | Other Industries | Fair Value, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2023 - $209.3; 2022 - $247.6)	7.5	9.2
Preferred Stocks | Other Industries | Fair Value, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2023 - $209.3; 2022 - $247.6)	2.4	1.6
Common Stock | Finance, Insurance and Real Estate | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2023 - $209.3; 2022 - $247.6)	0.6	0.9
Common Stock | Finance, Insurance and Real Estate | Fair Value, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2023 - $209.3; 2022 - $247.6)	0.6	0.9
Common Stock | Finance, Insurance and Real Estate | Fair Value, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2023 - $209.3; 2022 - $247.6)	0.0	0.0
Common Stock | Finance, Insurance and Real Estate | Fair Value, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2023 - $209.3; 2022 - $247.6)	0.0	0.0
Common Stock | Other Industries | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2023 - $209.3; 2022 - $247.6)	0.6	1.2
Common Stock | Other Industries | Fair Value, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2023 - $209.3; 2022 - $247.6)	0.2	0.3
Common Stock | Other Industries | Fair Value, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2023 - $209.3; 2022 - $247.6)	0.0	0.4
Common Stock | Other Industries | Fair Value, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2023 - $209.3; 2022 - $247.6)	0.4	0.5
Bond Exchange Traded Funds | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2023 - $209.3; 2022 - $247.6)	7.7	12.2
Bond Exchange Traded Funds | Fair Value, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2023 - $209.3; 2022 - $247.6)	7.7	12.2
Bond Exchange Traded Funds | Fair Value, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2023 - $209.3; 2022 - $247.6)	0.0
Bond Exchange Traded Funds | Fair Value, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2023 - $209.3; 2022 - $247.6)	0.0
Partnership Interest [Member] | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2023 - $209.3; 2022 - $247.6)	191.4	189.1
Partnership Interest [Member] | Fair Value, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2023 - $209.3; 2022 - $247.6)	0.0	0.0
Partnership Interest [Member] | Fair Value, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2023 - $209.3; 2022 - $247.6)	0.0	0.0
Partnership Interest [Member] | Fair Value, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2023 - $209.3; 2022 - $247.6)	0.0	0.0
Redeemable Preferred Stock | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)		8.0
Redeemable Preferred Stock | Fair Value, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)	0.0
Redeemable Preferred Stock | Fair Value, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)	1.2
Redeemable Preferred Stock | Fair Value, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)	7.1
Designated as Hedging Instrument | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Instruments Not Designated as Cash Flow Hedges		0.4
Designated as Hedging Instrument | Fair Value, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Instruments Not Designated as Cash Flow Hedges		0.0
Designated as Hedging Instrument | Fair Value, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Instruments Not Designated as Cash Flow Hedges		0.4
Designated as Hedging Instrument | Fair Value, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Instruments Not Designated as Cash Flow Hedges		0.0
Fair Value, Inputs, Net Asset Value | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Convertible Securities at Fair Value		0.0
Derivative Instruments Not Designated as Hedges	0.0	0.0
Total	191.4	189.1
Liabilities, Fair Value Disclosure		0.0
Fair Value, Inputs, Net Asset Value | Investments in Fixed Maturities | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)	0.0	0.0
Fair Value, Inputs, Net Asset Value | United States Government and Government Agencies and Authorities | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)	0.0	0.0
Fair Value, Inputs, Net Asset Value | States and Political Subdivisions | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)	0.0	0.0
Fair Value, Inputs, Net Asset Value | Debt Security, Government, Non-US [Member] | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)	0.0	0.0
Fair Value, Inputs, Net Asset Value | Corporate Debt Securities [Member] | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)	0.0	0.0
Fair Value, Inputs, Net Asset Value | Redeemable Preferred Stocks | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)		0.0
Fair Value, Inputs, Net Asset Value | Collateralized Loan Obligations | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)	0.0	0.0
Fair Value, Inputs, Net Asset Value | Other Mortgage- and Asset-backed | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)	0.0	0.0
Fair Value, Inputs, Net Asset Value | Dividends on Equity Securities Excluding Alternative Investments | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2023 - $209.3; 2022 - $247.6)	191.4	189.1
Fair Value, Inputs, Net Asset Value | Preferred Stocks | Finance, Insurance and Real Estate | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2023 - $209.3; 2022 - $247.6)	0.0	0.0
Fair Value, Inputs, Net Asset Value | Preferred Stocks | Other Industries | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2023 - $209.3; 2022 - $247.6)	0.0	0.0
Fair Value, Inputs, Net Asset Value | Common Stock | Finance, Insurance and Real Estate | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2023 - $209.3; 2022 - $247.6)	0.0	0.0
Fair Value, Inputs, Net Asset Value | Common Stock | Other Industries | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2023 - $209.3; 2022 - $247.6)	0.0	0.0
Fair Value, Inputs, Net Asset Value | Bond Exchange Traded Funds | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2023 - $209.3; 2022 - $247.6)	0.0	0.0
Fair Value, Inputs, Net Asset Value | Partnership Interest [Member] | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2023 - $209.3; 2022 - $247.6)	191.4	189.1
Fair Value, Inputs, Net Asset Value | Redeemable Preferred Stock | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2023 - $7,565.8; 2022 - $7,811.8 Allowance for Credit Losses: 2023 - $8.2; 2022 - $9.6)	$ 0.0
Fair Value, Inputs, Net Asset Value | Designated as Hedging Instrument | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Instruments Not Designated as Cash Flow Hedges		$ 0.0

[1] As of January 1, 2023, the Company adopted ASU 2018-12 using the modified retrospective method applied as of the transition date of January 1, 2021. Prior period amounts in the financial statements have been recast to reflect application of the new guidance. See Note 2 to the Consolidated Financial Statements for additional information.