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Investments - Narrative (Details) - USD ($) | 3 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | ---
Mar. 31, 2021 | Dec. 31, 2020 | Mar. 31, 2020 | Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date | $ 1,373,300,000 |  |  |
Fixed maturities, allowance for credit losses | 4,600,000 | $ 3,300,000 | $ 4,600,000 | $ 0
Equity securities | 897,400,000 | 858,500,000 |  |
Equity securities, FV-NI, unrealized gain (loss) | 56,500,000 |  |  |
Equity securities without readily determinable fair value, impairment loss, annual amount | 800,000 |  |  |
Equity securities without readily determinable fair value, downward price adjustment, cumulative amount | 500,000 |  |  |
Equity securities without readily determinable fair value, upward price adjustment, cumulative amount | 0 |  |  |
Equity securities without readily determinable fair value, impairment loss, cumulative amount | 6,700,000 |  |  |
Other Receivables | 225,400,000 | 222,400,000 |  |
Equity Securities at Fair Value (Cost: 2021 - $680.6; 2020 - $684.1) | 897,400,000 | 858,500,000 |  |
Reduction Due to Sales | (300,000) |  |  |
Variable Interest Entity, Not Primary Beneficiary |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount | 54,400,000 |  |  |
Variable Interest Entity, Funding Investment Commitment, Funding | 48,500,000 | 20,000,000.0 |  |
Government National Mortgage Association Certificates and Obligations (GNMA) |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date | 361,300,000 |  |  |
Federal National Mortgage Association Certificates and Obligations (FNMA) |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date | 4,800,000 |  |  |
Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC) |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date | 11,700,000 |  |  |
Other Non-Governmental Issuers |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date | 995,500,000 |  |  |
Investments in Fixed Maturities |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Debt securities, available-for-sale, unrealized loss position, accumulated loss | 55,200,000 | 31,700,000 |  |
12 Months or Longer | 16,000,000.0 | 20,000,000.0 |  |
Other Receivables | 23,700,000 | 5,100,000 |  |
Other liabilities | 54,700,000 | 4,300,000 |  |
Equity Securities |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Other Receivables | 0 |  |  |
Equity Securities | Fair Value, Measurements, Recurring |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Equity securities | 897,400,000 | 858,500,000 |  |
Equity Securities at Fair Value (Cost: 2021 - $680.6; 2020 - $684.1) | 897,400,000 | 858,500,000 |  |
Equity Securities | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Equity securities | 532,100,000 | 505,500,000 |  |
Equity Securities at Fair Value (Cost: 2021 - $680.6; 2020 - $684.1) | 532,100,000 | 505,500,000 |  |
Preferred Stocks | Other Industries | Fair Value, Measurements, Recurring |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Equity securities | 16,000,000.0 | 15,400,000 |  |
Equity Securities at Fair Value (Cost: 2021 - $680.6; 2020 - $684.1) | 16,000,000.0 | 15,400,000 |  |
Preferred Stocks | Other Industries | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Equity securities | 0 | 0 |  |
Equity Securities at Fair Value (Cost: 2021 - $680.6; 2020 - $684.1) | 0 | 0 |  |
Common Stock | Other Industries | Fair Value, Measurements, Recurring |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Equity securities | 2,900,000 | 400,000 |  |
Equity Securities at Fair Value (Cost: 2021 - $680.6; 2020 - $684.1) | 2,900,000 | 400,000 |  |
Common Stock | Other Industries | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Equity securities | 2,800,000 | 400,000 |  |
Equity Securities at Fair Value (Cost: 2021 - $680.6; 2020 - $684.1) | 2,800,000 | 400,000 |  |
Bonds and Notes |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Debt securities, available-for-sale, unrealized loss position, accumulated loss | 30,400,000 | 10,600,000 |  |
12 Months or Longer | 4,600,000 | 3,100,000 |  |
Fixed maturities, allowance for credit losses | 4,300,000 | 3,000,000.0 | $ 3,500,000 | $ 0
Reduction Due to Sales | (300,000) |  |  |
Variable Interest Entity, Not Primary Beneficiary | Equity Method Limited Liability Investments |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Outstanding commitments to fund equity method limited liability investments | 94,600,000 |  |  |
Variable Interest Entity, Not Primary Beneficiary | Alternative Energy Partnership Investments |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Outstanding commitments to fund equity method limited liability investments | 31,500,000 |  |  |
Investment-grade | Investments in Fixed Maturities |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Debt securities, available-for-sale, unrealized loss position, accumulated loss | 39,800,000 | 8,000,000.0 |  |
Non-investment-grade | Investments in Fixed Maturities |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Debt securities, available-for-sale, unrealized loss position, accumulated loss | $ 15,400,000 | $ 23,700,000 |  |
Percentage of unrealized loss position to amortized cost basis of available for sale security average | 9.00% | 11.00% |  |

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