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Fair Value Measurements - Valuation of Assets Measured at Fair Value (Details) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019
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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

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