Fair Value Measurements - Valuation of Assets Measured at Fair Value (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)	$ 6,894.8	$ 7,986.9
Equity Securities at Fair Value (Cost: 2022 - $247.6; 2021 - $618.7)	243.2	830.6
Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Convertible Securities at Fair Value	43.3	46.4
Derivative Instruments Not Designated as Hedges	1.7
Total	7,183.0	8,863.9
Liabilities, Fair Value Disclosure	(0.4)	0.0
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	0.0
Derivative Instruments Not Designated as Hedges	0.0
Total	117.0	586.6
Liabilities, Fair Value Disclosure	0.0	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	46.4
Derivative Instruments Not Designated as Hedges	1.7
Total	6,646.9	7,700.9
Derivative Instruments Designated as Cash Flow Hedges	(0.4)
Liabilities, Fair Value Disclosure	(0.4)	0.0
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	0.0
Derivative Instruments Not Designated as Hedges	0.0
Total	230.0	251.4
Liabilities, Fair Value Disclosure	0.0	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: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)	6,894.8	7,986.9
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: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)	103.6	132.8
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: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)	6,563.3	7,604.2
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: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)	227.9	249.9
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: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)	528.0	637.4
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: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)	528.0	637.4
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: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)	103.6	132.8
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: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)	424.4	504.6
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: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)	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: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)	1,568.9	1,890.1
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: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)	1,568.9	1,890.1
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: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)	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: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)	1,568.9	1,890.1
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: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)	0.0	0.0
Foreign Governments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)	4.1	5.5
Foreign Governments | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)	4.1	5.5
Foreign Governments | 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: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)	0.0	0.0
Foreign Governments | 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: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)	4.1	5.5
Foreign Governments | 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: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)	0.0	0.0
Corporate Bonds and Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)	3,539.4	4,386.9
Corporate Bonds and Notes | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)	3,539.4	4,386.9
Corporate Bonds and Notes | 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: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)	0.0	0.0
Corporate Bonds and Notes | 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: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)	3,323.4	4,150.1
Corporate Bonds and Notes | 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: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)	216.0	236.8
Redeemable Preferred Stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)	8.0	7.4
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: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)	8.0
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: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)		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: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)		1.3
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: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)		6.1
Collateralized Loan Obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)	953.9	752.1
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: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)	953.9	752.1
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: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)	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: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)	953.9	752.1
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: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)	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: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)	292.5	307.5
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: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)	292.5	307.5
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: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)	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: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)	287.4	300.5
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: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)	5.1	7.0
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: 2022 - $247.6; 2021 - $618.7)	243.2	830.6
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: 2022 - $247.6; 2021 - $618.7)	13.4	453.8
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: 2022 - $247.6; 2021 - $618.7)	38.6	50.3
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: 2022 - $247.6; 2021 - $618.7)	2.1	1.5
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: 2022 - $247.6; 2021 - $618.7)	29.0	34.2
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: 2022 - $247.6; 2021 - $618.7)	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: 2022 - $247.6; 2021 - $618.7)	29.0	34.2
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: 2022 - $247.6; 2021 - $618.7)	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: 2022 - $247.6; 2021 - $618.7)	10.8	17.6
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: 2022 - $247.6; 2021 - $618.7)	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: 2022 - $247.6; 2021 - $618.7)	9.2	16.1
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: 2022 - $247.6; 2021 - $618.7)	1.6	1.5
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: 2022 - $247.6; 2021 - $618.7)	0.9	18.9
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: 2022 - $247.6; 2021 - $618.7)	0.9	18.9
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: 2022 - $247.6; 2021 - $618.7)	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: 2022 - $247.6; 2021 - $618.7)	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: 2022 - $247.6; 2021 - $618.7)	1.2	2.9
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: 2022 - $247.6; 2021 - $618.7)	0.3	2.9
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: 2022 - $247.6; 2021 - $618.7)	0.4	0.0
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: 2022 - $247.6; 2021 - $618.7)	0.5	0.0
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: 2022 - $247.6; 2021 - $618.7)	12.2	432.0
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: 2022 - $247.6; 2021 - $618.7)	12.2	432.0
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: 2022 - $247.6; 2021 - $618.7)	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: 2022 - $247.6; 2021 - $618.7)	0.0
Limited Liability Companies and Limited Partnerships | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2022 - $247.6; 2021 - $618.7)	189.1	325.0
Limited Liability Companies and Limited Partnerships | 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: 2022 - $247.6; 2021 - $618.7)	0.0	0.0
Limited Liability Companies and Limited Partnerships | 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: 2022 - $247.6; 2021 - $618.7)	0.0	0.0
Limited Liability Companies and Limited Partnerships | 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: 2022 - $247.6; 2021 - $618.7)	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: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)		7.4
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: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)	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: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)	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: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)	6.8
Designated as Hedging Instrument | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Instruments Designated as Cash Flow Hedges	(0.4)	0.0
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 Designated as Cash Flow Hedges	0.0	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 Designated as Cash Flow Hedges		0.0
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 Designated as Cash Flow Hedges	0.0	0.0
Measured at 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	0.0
Derivative Instruments Not Designated as Hedges	0.0
Total	189.1	325.0
Liabilities, Fair Value Disclosure	0.0	0.0
Measured at 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: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)	0.0	0.0
Measured at 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: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)	0.0	0.0
Measured at 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: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)	0.0	0.0
Measured at Net Asset Value | Foreign Governments | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)	0.0	0.0
Measured at Net Asset Value | Corporate Bonds and Notes | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)	0.0	0.0
Measured at 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: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)		0.0
Measured at 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: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)	0.0	0.0
Measured at 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: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)	0.0	0.0
Measured at 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: 2022 - $247.6; 2021 - $618.7)	189.1	325.0
Measured at 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: 2022 - $247.6; 2021 - $618.7)	0.0	0.0
Measured at 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: 2022 - $247.6; 2021 - $618.7)	0.0	0.0
Measured at 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: 2022 - $247.6; 2021 - $618.7)	0.0	0.0
Measured at 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: 2022 - $247.6; 2021 - $618.7)	0.0	0.0
Measured at 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: 2022 - $247.6; 2021 - $618.7)	0.0	0.0
Measured at Net Asset Value | Limited Liability Companies and Limited Partnerships | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities at Fair Value (Cost: 2022 - $247.6; 2021 - $618.7)	189.1	325.0
Measured at 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: 2022 - $7,811.8; 2021 - $7,358.2 Allowance for Credit Losses: 2022 - $9.6; 2021 - $7.5)	0.0
Measured at 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 Designated as Cash Flow Hedges	$ 0.0	$ 0.0