Fair Value Measurements - Valuation of Assets Measured at Fair Value (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)	$ 7,986.9	$ 7,605.9
Equity Securities at Fair Value (Cost: 2021 - $618.7; 2020 - $684.1)	830.6	858.5
Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Convertible securities at fair value	46.4	39.9
Total	8,863.9	8,504.3
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]
Total	586.6	639.5
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	46.4	39.9
Total	7,700.9	7,123.4
Fair Value, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	251.4	449.2
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: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)	7,986.9	7,605.9
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: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)	132.8	134.0
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: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)	7,604.2	7,022.7
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: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)	249.9	449.2
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: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)	637.4	585.3
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: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)	637.4	585.3
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: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)	132.8	134.0
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: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)	504.6	451.3
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: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)	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: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)	1,890.1	1,589.5
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: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)	1,890.1	1,589.5
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: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)	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: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)	1,890.1	1,589.5
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: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)	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: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)	5.5	5.2
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: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)	5.5	5.2
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: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)	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: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)	5.5	5.2
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: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)	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: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)	4,386.9	4,425.4
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: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)	4,386.9	4,425.4
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: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)	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: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)	4,150.1	3,992.4
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: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)	236.8	433.0
Redeemable Preferred Stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)	7.4	7.5
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: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)	7.4
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: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)		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: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)		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: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)		6.2
Collateralized Loan Obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)	752.1	767.7
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: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)	752.1	767.7
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: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)	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: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)	752.1	767.7
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: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)	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: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)	307.5	225.3
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: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)	307.5	225.3
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: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)	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: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)	300.5	215.3
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: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)	7.0	10.0
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: 2021 - $618.7; 2020 - $684.1)	830.6	858.5
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: 2021 - $618.7; 2020 - $684.1)	453.8	505.5
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: 2021 - $618.7; 2020 - $684.1)	50.3	60.8
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: 2021 - $618.7; 2020 - $684.1)	1.5	0.0
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: 2021 - $618.7; 2020 - $684.1)	34.2	43.7
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: 2021 - $618.7; 2020 - $684.1)	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: 2021 - $618.7; 2020 - $684.1)	34.2	43.7
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: 2021 - $618.7; 2020 - $684.1)	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: 2021 - $618.7; 2020 - $684.1)	17.6	15.4
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: 2021 - $618.7; 2020 - $684.1)	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: 2021 - $618.7; 2020 - $684.1)	16.1	15.4
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: 2021 - $618.7; 2020 - $684.1)	1.5	0.0
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: 2021 - $618.7; 2020 - $684.1)	18.9	10.4
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: 2021 - $618.7; 2020 - $684.1)	18.9	8.7
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: 2021 - $618.7; 2020 - $684.1)	0.0	1.7
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: 2021 - $618.7; 2020 - $684.1)	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: 2021 - $618.7; 2020 - $684.1)	2.9	0.4
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: 2021 - $618.7; 2020 - $684.1)	2.9	0.4
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: 2021 - $618.7; 2020 - $684.1)		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: 2021 - $618.7; 2020 - $684.1)		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: 2021 - $618.7; 2020 - $684.1)	432.0	496.4
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: 2021 - $618.7; 2020 - $684.1)	432.0	496.4
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: 2021 - $618.7; 2020 - $684.1)	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: 2021 - $618.7; 2020 - $684.1)	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: 2021 - $618.7; 2020 - $684.1)	325.0	292.2
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: 2021 - $618.7; 2020 - $684.1)	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: 2021 - $618.7; 2020 - $684.1)	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: 2021 - $618.7; 2020 - $684.1)	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: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)		7.5
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: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)	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: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)	1.3
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: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)	6.1
Measured at Net Asset Value | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	325.0	292.2
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: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)	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: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)	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: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)	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: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)	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: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)	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: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)		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: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)	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: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)	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: 2021 - $618.7; 2020 - $684.1)	325.0	292.2
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: 2021 - $618.7; 2020 - $684.1)	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: 2021 - $618.7; 2020 - $684.1)	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: 2021 - $618.7; 2020 - $684.1)	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: 2021 - $618.7; 2020 - $684.1)		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: 2021 - $618.7; 2020 - $684.1)	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: 2021 - $618.7; 2020 - $684.1)	325.0	$ 292.2
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: 2021 - $7,358.2; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $7.5; 2020 - $3.3)	$ 0.0