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Percentage of Unrealized Loss Position to Amortized Cost Basis of Available for Sale Security Average

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Variable Interest Entity, Funding Investment Commitment, Funding

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Fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), without single maturity date and not allocated over maturity grouping.

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Amount of allowance for credit loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of decrease in allowance for credit loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale) from sale.

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Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Amount of unrealized gain (loss) on investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Amount of cumulative loss from downward price adjustment on investment in equity security without readily determinable fair value.

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Amount of impairment loss on investment in equity security without readily determinable fair value.

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Amount of cumulative impairment loss on investment in equity security without readily determinable fair value.

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Amount of cumulative gain from upward price adjustment on investment in equity security without readily determinable fair value.

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Amount of liabilities classified as other.

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Amount due from parties in nontrade transactions, classified as other.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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The reporting entity's maximum amount of exposure to loss as a result of its involvement with the Variable Interest Entity (VIE).

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Investments - Narrative (Details) - USD ($)	3 Months Ended	12 Months Ended
Investments - Narrative (Details) - USD ($)	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%