Investments - Narrative (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date	$ 1,367,400,000		$ 1,367,400,000
Fixed maturities, allowance for credit losses	4,400,000	$ 4,800,000	4,400,000	$ 4,800,000	$ 3,300,000	$ 0
Equity securities	969,600,000		969,600,000		858,500,000
Equity securities, FV-NI, unrealized gain (loss)	107,300,000
Equity securities without readily determinable fair value, impairment loss, annual amount	700,000		4,100,000
Equity securities without readily determinable fair value, downward price adjustment, cumulative amount	500,000		500,000
Equity securities without readily determinable fair value, upward price adjustment, cumulative amount	0		0
Equity securities without readily determinable fair value, impairment loss, cumulative amount	9,900,000		9,900,000
Other Receivables	207,300,000		207,300,000		222,400,000
Equity Securities at Fair Value (Cost: 2021 - $725.1; 2020 - $684.1)	969,600,000		969,600,000		858,500,000
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	293,800,000		293,800,000
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	28,800,000		28,800,000
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	26,000,000		26,000,000
Other Non-Governmental Issuers
Debt Securities, Available-for-sale [Line Items]
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date	1,018,800,000		1,018,800,000
Investments in Fixed Maturities
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, unrealized loss position, accumulated loss	34,900,000		34,900,000		31,700,000
12 Months or Longer	4,800,000		4,800,000		20,000,000.0
Other Receivables	6,800,000		6,800,000		5,100,000
Other liabilities	38,700,000		38,700,000		4,300,000
Equity Securities
Debt Securities, Available-for-sale [Line Items]
Other Receivables	200,000		200,000
Other liabilities	0		0
Equity Securities | Fair Value, Measurements, Recurring
Debt Securities, Available-for-sale [Line Items]
Equity securities	969,600,000		969,600,000		858,500,000
Equity Securities at Fair Value (Cost: 2021 - $725.1; 2020 - $684.1)	969,600,000		969,600,000		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	589,000,000.0		589,000,000.0		505,500,000
Equity Securities at Fair Value (Cost: 2021 - $725.1; 2020 - $684.1)	589,000,000.0		589,000,000.0		505,500,000
Preferred Stocks | Other Industries | Fair Value, Measurements, Recurring
Debt Securities, Available-for-sale [Line Items]
Equity securities	17,100,000		17,100,000		15,400,000
Equity Securities at Fair Value (Cost: 2021 - $725.1; 2020 - $684.1)	17,100,000		17,100,000		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		0
Equity Securities at Fair Value (Cost: 2021 - $725.1; 2020 - $684.1)	0		0		0
Common Stock | Other Industries | Fair Value, Measurements, Recurring
Debt Securities, Available-for-sale [Line Items]
Equity securities	3,100,000		3,100,000		400,000
Equity Securities at Fair Value (Cost: 2021 - $725.1; 2020 - $684.1)	3,100,000		3,100,000		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	3,100,000		3,100,000		400,000
Equity Securities at Fair Value (Cost: 2021 - $725.1; 2020 - $684.1)	3,100,000		3,100,000		400,000
Bonds and Notes
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, unrealized loss position, accumulated loss	15,800,000		15,800,000		10,600,000
12 Months or Longer	1,800,000		1,800,000		3,100,000
Fixed maturities, allowance for credit losses	4,400,000	4,200,000	4,400,000	4,200,000	3,000,000.0	$ 0
Variable Interest Entity, Not Primary Beneficiary
Debt Securities, Available-for-sale [Line Items]
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount	54,200,000		54,200,000
Variable Interest Entity, Funding Investment Commitment, Funding	31,500,000	$ 0	80,000,000.0	$ 0
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	83,700,000		83,700,000
Investment-grade | Investments in Fixed Maturities
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, unrealized loss position, accumulated loss	27,200,000		27,200,000		8,000,000
Non-investment-grade | Investments in Fixed Maturities
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, unrealized loss position, accumulated loss	$ 7,700,000		$ 7,700,000		$ 23,700,000
Percentage of unrealized loss position to amortized cost basis of available for sale security average	5.00%		5.00%		11.00%