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Fair Value Measurements - Valuation of Assets Measured at Fair Value (Details) - USD ($) | Dec. 31, 2019 | Dec. 31, 2018
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5) | $ 6,922,100,000 | $ 6,424,200,000
Equity Securities at Fair Value (Cost: 2019 - $818.8; 2018 - $720.8) | 907,300,000 | 684,400,000
Fair Value, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Convertible securities at fair value | 37,300,000 | 31,500,000
Total | 7,866,700,000 | 7,140,100,000
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
Total | 745,000,000.0 | 594,200,000
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 | 37,300,000 | 31,500,000
Total | 5,829,400,000 | 5,456,500,000
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
Total | 1,044,200,000 | 897,400,000
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: 2019 - $6,372.7; 2018 - $6,284.5) | 6,922,100,000 | 6,424,200,000
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: 2019 - $6,372.7; 2018 - $6,284.5) | 144,300,000 | 156,500,000
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: 2019 - $6,372.7; 2018 - $6,284.5) | 5,733,600,000 | 5,370,300,000
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: 2019 - $6,372.7; 2018 - $6,284.5) | 1,044,200,000 | 897,400,000
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: 2019 - $6,372.7; 2018 - $6,284.5) | 815,900,000 | 865,700,000
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: 2019 - $6,372.7; 2018 - $6,284.5) | 815,900,000 | 865,700,000
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: 2019 - $6,372.7; 2018 - $6,284.5) | 144,300,000 | 156,500,000
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: 2019 - $6,372.7; 2018 - $6,284.5) | 671,600,000 | 709,200,000
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: 2019 - $6,372.7; 2018 - $6,284.5) | 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: 2019 - $6,372.7; 2018 - $6,284.5) | 1,515,800,000 | 1,619,100,000
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: 2019 - $6,372.7; 2018 - $6,284.5) | 1,515,800,000 | 1,619,100,000
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: 2019 - $6,372.7; 2018 - $6,284.5) | 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: 2019 - $6,372.7; 2018 - $6,284.5) | 1,515,800,000 | 1,619,100,000
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: 2019 - $6,372.7; 2018 - $6,284.5) | 0 | 0
Foreign Governments |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5) | 16,800,000 | 5,900,000
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: 2019 - $6,372.7; 2018 - $6,284.5) | 16,800,000 | 5,900,000
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: 2019 - $6,372.7; 2018 - $6,284.5) | 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: 2019 - $6,372.7; 2018 - $6,284.5) | 16,800,000 | 5,900,000
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: 2019 - $6,372.7; 2018 - $6,284.5) | 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: 2019 - $6,372.7; 2018 - $6,284.5) | 3,859,700,000 | 3,393,800,000
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: 2019 - $6,372.7; 2018 - $6,284.5) | 3,859,700,000 | 3,393,800,000
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: 2019 - $6,372.7; 2018 - $6,284.5) | 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: 2019 - $6,372.7; 2018 - $6,284.5) | 3,450,600,000 | 3,011,200,000
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: 2019 - $6,372.7; 2018 - $6,284.5) | 409,100,000 | 382,600,000
Redeemable Preferred Stocks |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5) | 6,700,000 |
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: 2019 - $6,372.7; 2018 - $6,284.5) | 6,700,000 |
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: 2019 - $6,372.7; 2018 - $6,284.5) | 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: 2019 - $6,372.7; 2018 - $6,284.5) | 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: 2019 - $6,372.7; 2018 - $6,284.5) | 6,700,000 |
Collateralized Loan Obligations |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed Maturities at Fair Value (Amortized Cost: 2019 - $6,372.7; 2018 - $6,284.5) | 618,200,000 | 524,000,000.0
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: 2019 - $6,372.7; 2018 - $6,284.5) | 618,200,000 | 524,000,000.0
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: 2019 - $6,372.7; 2018 - $6,284.5) | 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: 2019 - $6,372.7; 2018 - $6,284.5) | 0 | 19,100,000
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: 2019 - $6,372.7; 2018 - $6,284.5) | 618,200,000 | 504,900,000
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: 2019 - $6,372.7; 2018 - $6,284.5) | 89,000,000.0 | 15,700,000
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: 2019 - $6,372.7; 2018 - $6,284.5) | 89,000,000.0 | 15,700,000
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: 2019 - $6,372.7; 2018 - $6,284.5) | 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: 2019 - $6,372.7; 2018 - $6,284.5) | 78,800,000 | 5,800,000
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: 2019 - $6,372.7; 2018 - $6,284.5) | 10,200,000 | 9,900,000
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: 2019 - $818.8; 2018 - $720.8) | 907,300,000 | 684,400,000
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: 2019 - $818.8; 2018 - $720.8) | 600,700,000 | 437,700,000
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: 2019 - $818.8; 2018 - $720.8) | 58,500,000 | 54,700,000
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: 2019 - $818.8; 2018 - $720.8) | 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: 2019 - $818.8; 2018 - $720.8) | 44,500,000 | 41,200,000
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: 2019 - $818.8; 2018 - $720.8) | 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: 2019 - $818.8; 2018 - $720.8) | 44,500,000 | 41,200,000
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: 2019 - $818.8; 2018 - $720.8) | 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: 2019 - $818.8; 2018 - $720.8) | 14,700,000 | 13,000,000.0
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: 2019 - $818.8; 2018 - $720.8) | 900,000 | 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: 2019 - $818.8; 2018 - $720.8) | 13,800,000 | 13,000,000.0
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: 2019 - $818.8; 2018 - $720.8) | 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: 2019 - $818.8; 2018 - $720.8) | 12,800,000 | 10,200,000
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: 2019 - $818.8; 2018 - $720.8) | 12,800,000 | 10,200,000
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: 2019 - $818.8; 2018 - $720.8) | 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: 2019 - $818.8; 2018 - $720.8) | 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: 2019 - $818.8; 2018 - $720.8) | 400,000 | 700,000
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: 2019 - $818.8; 2018 - $720.8) | 200,000 | 200,000
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: 2019 - $818.8; 2018 - $720.8) | 200,000 | 500,000
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: 2019 - $818.8; 2018 - $720.8) | 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: 2019 - $818.8; 2018 - $720.8) | 586,800,000 | 427,300,000
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: 2019 - $818.8; 2018 - $720.8) | 586,800,000 | 427,300,000
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: 2019 - $818.8; 2018 - $720.8) | 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: 2019 - $818.8; 2018 - $720.8) | 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: 2019 - $818.8; 2018 - $720.8) | 248,100,000 | 192,000,000.0
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: 2019 - $818.8; 2018 - $720.8) | 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: 2019 - $818.8; 2018 - $720.8) | 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: 2019 - $818.8; 2018 - $720.8) | 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
Total | 248,100,000 | 192,000,000.0
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: 2019 - $6,372.7; 2018 - $6,284.5) | 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: 2019 - $6,372.7; 2018 - $6,284.5) | 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: 2019 - $6,372.7; 2018 - $6,284.5) | 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: 2019 - $6,372.7; 2018 - $6,284.5) | 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: 2019 - $6,372.7; 2018 - $6,284.5) | 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: 2019 - $6,372.7; 2018 - $6,284.5) | 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: 2019 - $6,372.7; 2018 - $6,284.5) | 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: 2019 - $6,372.7; 2018 - $6,284.5) | 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: 2019 - $818.8; 2018 - $720.8) | 248,100,000 | 192,000,000.0
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: 2019 - $818.8; 2018 - $720.8) | 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: 2019 - $818.8; 2018 - $720.8) | 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: 2019 - $818.8; 2018 - $720.8) | 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: 2019 - $818.8; 2018 - $720.8) | 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: 2019 - $818.8; 2018 - $720.8) | 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: 2019 - $818.8; 2018 - $720.8) | $ 248,100,000 | $ 192,000,000.0

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