Fair Value Measurements - Valuation of Assets Measured at Fair Value (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2020 - $6,692.7; 2019 - $6,372.7; Allowance for Credit Losses: 2020 - $3.3)	$ 7,605.9	$ 6,922.1
Equity Securities at Fair Value (Cost: 2020 - $684.1; 2019 - $818.8)	858.5	907.3
Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Convertible securities at fair value	39.9	37.3
Total	8,504.3	7,866.7
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	639.5	745.0
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	39.9	37.3
Total	7,123.4	5,829.4
Fair Value, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	449.2	1,044.2
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: 2020 - $6,692.7; 2019 - $6,372.7; Allowance for Credit Losses: 2020 - $3.3)	7,605.9	6,922.1
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: 2020 - $6,692.7; 2019 - $6,372.7; Allowance for Credit Losses: 2020 - $3.3)	134.0	144.3
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: 2020 - $6,692.7; 2019 - $6,372.7; Allowance for Credit Losses: 2020 - $3.3)	7,022.7	5,733.6
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: 2020 - $6,692.7; 2019 - $6,372.7; Allowance for Credit Losses: 2020 - $3.3)	449.2	1,044.2
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: 2020 - $6,692.7; 2019 - $6,372.7; Allowance for Credit Losses: 2020 - $3.3)	585.3	815.9
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: 2020 - $6,692.7; 2019 - $6,372.7; Allowance for Credit Losses: 2020 - $3.3)	585.3	815.9
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: 2020 - $6,692.7; 2019 - $6,372.7; Allowance for Credit Losses: 2020 - $3.3)	134.0	144.3
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: 2020 - $6,692.7; 2019 - $6,372.7; Allowance for Credit Losses: 2020 - $3.3)	451.3	671.6
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: 2020 - $6,692.7; 2019 - $6,372.7; Allowance for Credit Losses: 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: 2020 - $6,692.7; 2019 - $6,372.7; Allowance for Credit Losses: 2020 - $3.3)	1,589.5	1,515.8
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: 2020 - $6,692.7; 2019 - $6,372.7; Allowance for Credit Losses: 2020 - $3.3)	1,589.5	1,515.8
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: 2020 - $6,692.7; 2019 - $6,372.7; Allowance for Credit Losses: 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: 2020 - $6,692.7; 2019 - $6,372.7; Allowance for Credit Losses: 2020 - $3.3)	1,589.5	1,515.8
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: 2020 - $6,692.7; 2019 - $6,372.7; Allowance for Credit Losses: 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: 2020 - $6,692.7; 2019 - $6,372.7; Allowance for Credit Losses: 2020 - $3.3)	5.2	16.8
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: 2020 - $6,692.7; 2019 - $6,372.7; Allowance for Credit Losses: 2020 - $3.3)	5.2	16.8
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: 2020 - $6,692.7; 2019 - $6,372.7; Allowance for Credit Losses: 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: 2020 - $6,692.7; 2019 - $6,372.7; Allowance for Credit Losses: 2020 - $3.3)	5.2	16.8
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: 2020 - $6,692.7; 2019 - $6,372.7; Allowance for Credit Losses: 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: 2020 - $6,692.7; 2019 - $6,372.7; Allowance for Credit Losses: 2020 - $3.3)	4,425.4	3,859.7
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: 2020 - $6,692.7; 2019 - $6,372.7; Allowance for Credit Losses: 2020 - $3.3)	4,425.4	3,859.7
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: 2020 - $6,692.7; 2019 - $6,372.7; Allowance for Credit Losses: 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: 2020 - $6,692.7; 2019 - $6,372.7; Allowance for Credit Losses: 2020 - $3.3)	3,992.4	3,450.6
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: 2020 - $6,692.7; 2019 - $6,372.7; Allowance for Credit Losses: 2020 - $3.3)	433.0	409.1
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: 2020 - $6,692.7; 2019 - $6,372.7; Allowance for Credit Losses: 2020 - $3.3)	7.5	6.7
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: 2020 - $6,692.7; 2019 - $6,372.7; Allowance for Credit Losses: 2020 - $3.3)	0.0	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: 2020 - $6,692.7; 2019 - $6,372.7; Allowance for Credit Losses: 2020 - $3.3)	1.3	0.0
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: 2020 - $6,692.7; 2019 - $6,372.7; Allowance for Credit Losses: 2020 - $3.3)	6.2	6.7
Collateralized Loan Obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities at Fair Value (Amortized Cost: 2020 - $6,692.7; 2019 - $6,372.7; Allowance for Credit Losses: 2020 - $3.3)	767.7	618.2
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: 2020 - $6,692.7; 2019 - $6,372.7; Allowance for Credit Losses: 2020 - $3.3)	767.7	618.2
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: 2020 - $6,692.7; 2019 - $6,372.7; Allowance for Credit Losses: 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: 2020 - $6,692.7; 2019 - $6,372.7; Allowance for Credit Losses: 2020 - $3.3)	767.7	0.0
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: 2020 - $6,692.7; 2019 - $6,372.7; Allowance for Credit Losses: 2020 - $3.3)	0.0	618.2
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: 2020 - $6,692.7; 2019 - $6,372.7; Allowance for Credit Losses: 2020 - $3.3)	225.3	89.0
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: 2020 - $6,692.7; 2019 - $6,372.7; Allowance for Credit Losses: 2020 - $3.3)	225.3	89.0
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: 2020 - $6,692.7; 2019 - $6,372.7; Allowance for Credit Losses: 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: 2020 - $6,692.7; 2019 - $6,372.7; Allowance for Credit Losses: 2020 - $3.3)	215.3	78.8
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: 2020 - $6,692.7; 2019 - $6,372.7; Allowance for Credit Losses: 2020 - $3.3)	10.0	10.2
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: 2020 - $684.1; 2019 - $818.8)	858.5	907.3
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: 2020 - $684.1; 2019 - $818.8)	505.5	600.7
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: 2020 - $684.1; 2019 - $818.8)	60.8	58.5
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: 2020 - $684.1; 2019 - $818.8)	0.0	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: 2020 - $684.1; 2019 - $818.8)	43.7	44.5
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: 2020 - $684.1; 2019 - $818.8)	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: 2020 - $684.1; 2019 - $818.8)	43.7	44.5
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: 2020 - $684.1; 2019 - $818.8)	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: 2020 - $684.1; 2019 - $818.8)	15.4	14.7
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: 2020 - $684.1; 2019 - $818.8)	0.0	0.9
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: 2020 - $684.1; 2019 - $818.8)	15.4	13.8
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: 2020 - $684.1; 2019 - $818.8)	0.0	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: 2020 - $684.1; 2019 - $818.8)	10.4	12.8
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: 2020 - $684.1; 2019 - $818.8)	8.7	12.8
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: 2020 - $684.1; 2019 - $818.8)	1.7	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: 2020 - $684.1; 2019 - $818.8)	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: 2020 - $684.1; 2019 - $818.8)	0.4	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: 2020 - $684.1; 2019 - $818.8)	0.4	0.2
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: 2020 - $684.1; 2019 - $818.8)	0.0	0.2
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: 2020 - $684.1; 2019 - $818.8)	0.0	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: 2020 - $684.1; 2019 - $818.8)	496.4	586.8
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: 2020 - $684.1; 2019 - $818.8)	496.4	586.8
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: 2020 - $684.1; 2019 - $818.8)	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: 2020 - $684.1; 2019 - $818.8)	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: 2020 - $684.1; 2019 - $818.8)	292.2	248.1
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: 2020 - $684.1; 2019 - $818.8)	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: 2020 - $684.1; 2019 - $818.8)	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: 2020 - $684.1; 2019 - $818.8)	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]
Total	292.2	248.1
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: 2020 - $6,692.7; 2019 - $6,372.7; Allowance for Credit Losses: 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: 2020 - $6,692.7; 2019 - $6,372.7; Allowance for Credit Losses: 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: 2020 - $6,692.7; 2019 - $6,372.7; Allowance for Credit Losses: 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: 2020 - $6,692.7; 2019 - $6,372.7; Allowance for Credit Losses: 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: 2020 - $6,692.7; 2019 - $6,372.7; Allowance for Credit Losses: 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: 2020 - $6,692.7; 2019 - $6,372.7; Allowance for Credit Losses: 2020 - $3.3)	0.0	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: 2020 - $6,692.7; 2019 - $6,372.7; Allowance for Credit Losses: 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: 2020 - $6,692.7; 2019 - $6,372.7; Allowance for Credit Losses: 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: 2020 - $684.1; 2019 - $818.8)	292.2	248.1
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: 2020 - $684.1; 2019 - $818.8)	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: 2020 - $684.1; 2019 - $818.8)	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: 2020 - $684.1; 2019 - $818.8)	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: 2020 - $684.1; 2019 - $818.8)	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: 2020 - $684.1; 2019 - $818.8)	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: 2020 - $684.1; 2019 - $818.8)	$ 292.2	$ 248.1