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Investments - Schedule of Fixed Maturities (Details) - USD ($) | Mar. 31, 2021 | Dec. 31, 2020 | Mar. 31, 2020 | Dec. 31, 2019
--- | --- | --- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Fixed maturities, at amortized cost | $ 6,929,600,000 | $ 6,692,700,000 |  |
Gross unrealized gains | 609,600,000 |  |  |
Gross unrealized loss | (55,200,000) |  |  |
Fixed maturities, allowance for credit losses | (4,600,000) | (3,300,000) | $ (4,600,000) | $ 0
Fixed maturities at fair value | 7,479,400,000 | 7,605,900,000 |  |
Investments in Fixed Maturities |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Fixed maturities, at amortized cost |  | 6,692,700,000 |  |
Gross unrealized gains |  | 948,200,000 |  |
Gross unrealized loss |  | (31,700,000) |  |
Fixed maturities at fair value |  | 7,605,900,000 |  |
Debt securities, available-for-sale, unrealized loss position, accumulated loss | 55,200,000 | 31,700,000 |  |
Investments in Fixed Maturities | Investment-grade |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Debt securities, available-for-sale, unrealized loss position, accumulated loss | 39,800,000 | 8,000,000.0 |  |
U.S. Government and Government Agencies and Authorities |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Fixed maturities, at amortized cost | 498,000,000.0 | 536,500,000 |  |
Gross unrealized gains | 35,700,000 | 48,900,000 |  |
Gross unrealized loss | (1,300,000) | (100,000) |  |
Fixed maturities, allowance for credit losses | 0 |  |  |
Fixed maturities at fair value | 532,400,000 | 585,300,000 |  |
Debt securities, available-for-sale, unrealized loss position, accumulated loss | 1,300,000 | 100,000 |  |
States and Political Subdivisions |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Fixed maturities, at amortized cost | 1,538,600,000 | 1,404,300,000 |  |
Gross unrealized gains | 140,900,000 | 185,400,000 |  |
Gross unrealized loss | (11,600,000) | (200,000) |  |
Fixed maturities, allowance for credit losses | 0 |  |  |
Fixed maturities at fair value | 1,667,900,000 | 1,589,500,000 |  |
Debt securities, available-for-sale, unrealized loss position, accumulated loss | 11,600,000 | 200,000 |  |
Foreign Governments |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Fixed maturities, at amortized cost | 6,300,000 | 6,600,000 |  |
Gross unrealized gains | 0 | 0 |  |
Gross unrealized loss | (1,200,000) | (1,100,000) |  |
Fixed maturities, allowance for credit losses | (300,000) | (300,000) | (1,100,000) | 0
Fixed maturities at fair value | 4,800,000 | 5,200,000 |  |
Debt securities, available-for-sale, unrealized loss position, accumulated loss | 1,200,000 | 1,100,000 |  |
Bonds and Notes |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Fixed maturities, at amortized cost | 3,886,300,000 | 3,749,500,000 |  |
Gross unrealized gains | 419,200,000 | 689,500,000 |  |
Gross unrealized loss | (30,400,000) | (10,600,000) |  |
Fixed maturities, allowance for credit losses | (4,300,000) | (3,000,000.0) | $ (3,500,000) | $ 0
Fixed maturities at fair value | 4,270,800,000 | 4,425,400,000 |  |
Debt securities, available-for-sale, unrealized loss position, accumulated loss | 30,400,000 | 10,600,000 |  |
Redeemable Preferred Stocks |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Fixed maturities, at amortized cost | 7,000,000.0 | 7,000,000.0 |  |
Gross unrealized gains | 100,000 | 500,000 |  |
Gross unrealized loss | (100,000) | 0 |  |
Fixed maturities, allowance for credit losses | 0 |  |  |
Fixed maturities at fair value | 7,000,000.0 | 7,500,000 |  |
Debt securities, available-for-sale, unrealized loss position, accumulated loss | 100,000 |  |  |
Collateralized Loan Obligations |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Fixed maturities, at amortized cost | 791,000,000.0 | 785,100,000 |  |
Gross unrealized gains | 2,400,000 | 2,300,000 |  |
Gross unrealized loss | (10,500,000) | (19,700,000) |  |
Fixed maturities, allowance for credit losses | 0 |  |  |
Fixed maturities at fair value | 782,900,000 | 767,700,000 |  |
Debt securities, available-for-sale, unrealized loss position, accumulated loss | 10,500,000 | 19,700,000 |  |
Other Mortgage- and Asset-backed |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Fixed maturities, at amortized cost | 202,400,000 | 203,700,000 |  |
Gross unrealized gains | 11,300,000 | 21,600,000 |  |
Gross unrealized loss | (100,000) | 0 |  |
Fixed maturities, allowance for credit losses | 0 |  |  |
Fixed maturities at fair value | 213,600,000 | 225,300,000 |  |
Debt securities, available-for-sale, unrealized loss position, accumulated loss | $ 100,000 | $ 0 |  |

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