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Investments - Amortized Cost and Estimated Fair Value of investments (Details) - USD ($) | Dec. 31, 2020 | Dec. 31, 2019
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Debt Securities, Available-for-sale [Line Items] |  |
Amortized cost of fixed maturities | $ 6,692,700,000 | $ 6,372,700,000
Gross Unrealized Gains | 948,200,000 | 570,200,000
Gross Unrealized Losses | (31,700,000) | (20,800,000)
Fair Value | 7,605,900,000 | 6,922,100,000
Amortized cost of fixed maturities | (3,300,000) | 0
United States Government and Government Agencies and Authorities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized cost of fixed maturities | 536,500,000 | 784,700,000
Gross Unrealized Gains | 48,900,000 | 32,500,000
Gross Unrealized Losses | (100,000) | (1,300,000)
Fair Value | 585,300,000 | 815,900,000
Amortized cost of fixed maturities | 0 |
States and Political Subdivisions |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized cost of fixed maturities | 1,404,300,000 | 1,386,400,000
Gross Unrealized Gains | 185,400,000 | 130,500,000
Gross Unrealized Losses | (200,000) | (1,100,000)
Fair Value | 1,589,500,000 | 1,515,800,000
Amortized cost of fixed maturities | 0 |
Foreign Governments |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized cost of fixed maturities | 6,600,000 | 17,200,000
Gross Unrealized Gains | 0 | 1,200,000
Gross Unrealized Losses | (1,100,000) | (1,600,000)
Fair Value | 5,200,000 | 16,800,000
Amortized cost of fixed maturities | (300,000) | 0
Corporate Bonds and Notes |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized cost of fixed maturities | 3,749,500,000 | 3,465,000,000.0
Gross Unrealized Gains | 689,500,000 | 401,800,000
Gross Unrealized Losses | (10,600,000) | (7,100,000)
Fair Value | 4,425,400,000 | 3,859,700,000
Amortized cost of fixed maturities | (3,000,000.0) | 0
Redeemable Preferred Stocks |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized cost of fixed maturities | 7,000,000.0 | 6,800,000
Gross Unrealized Gains | 500,000 | 0
Gross Unrealized Losses | 0 | (100,000)
Fair Value | 7,500,000 | 6,700,000
Amortized cost of fixed maturities | 0 |
Collateralized Loan Obligations |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized cost of fixed maturities | 785,100,000 | 624,600,000
Gross Unrealized Gains | 2,300,000 | 2,100,000
Gross Unrealized Losses | (19,700,000) | (8,500,000)
Fair Value | 767,700,000 | 618,200,000
Amortized cost of fixed maturities | 0 |
Other Mortgage- and Asset-backed |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized cost of fixed maturities | 203,700,000 | 88,000,000.0
Gross Unrealized Gains | 21,600,000 | 2,100,000
Gross Unrealized Losses | 0 | (1,100,000)
Fair Value | 225,300,000 | $ 89,000,000.0
Amortized cost of fixed maturities | $ 0 |

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