Investments - Narrative (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 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	$ 1,382,500,000		$ 1,382,500,000
Fixed maturities, allowance for credit losses	5,100,000	$ 5,400,000	5,100,000	$ 5,400,000	$ 3,300,000	$ 0
Equity securities	957,700,000		957,700,000		858,500,000
Equity securities, FV-NI, unrealized gain (loss)	99,300,000
Equity securities without readily determinable fair value, impairment loss, annual amount	3,400,000
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	9,300,000		9,300,000
Other Receivables	214,900,000		214,900,000		222,400,000
Equity Securities at Fair Value (Cost: 2021 - $715.8; 2020 - $684.1)	957,700,000		957,700,000		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	321,400,000		321,400,000
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	15,000,000.0		15,000,000.0
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	28,300,000		28,300,000
Other Non-Governmental Issuers
Debt Securities, Available-for-sale [Line Items]
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date	1,017,800,000		1,017,800,000
Investments in Fixed Maturities
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, unrealized loss position, accumulated loss	20,000,000.0		20,000,000.0		31,700,000
12 Months or Longer	7,300,000		7,300,000		20,000,000.0
Other Receivables	17,100,000		17,100,000		5,100,000
Other liabilities	34,600,000		34,600,000		4,300,000
Equity Securities
Debt Securities, Available-for-sale [Line Items]
Other Receivables	400,000		400,000
Other liabilities	0		0
Equity Securities | Fair Value, Measurements, Recurring
Debt Securities, Available-for-sale [Line Items]
Equity securities	957,700,000		957,700,000		858,500,000
Equity Securities at Fair Value (Cost: 2021 - $715.8; 2020 - $684.1)	957,700,000		957,700,000		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	587,100,000		587,100,000		505,500,000
Equity Securities at Fair Value (Cost: 2021 - $715.8; 2020 - $684.1)	587,100,000		587,100,000		505,500,000
Preferred Stocks | Other Industries | Fair Value, Measurements, Recurring
Debt Securities, Available-for-sale [Line Items]
Equity securities	17,600,000		17,600,000		15,400,000
Equity Securities at Fair Value (Cost: 2021 - $715.8; 2020 - $684.1)	17,600,000		17,600,000		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 - $715.8; 2020 - $684.1)	0		0		0
Common Stock | Other Industries | Fair Value, Measurements, Recurring
Debt Securities, Available-for-sale [Line Items]
Equity securities	2,900,000		2,900,000		400,000
Equity Securities at Fair Value (Cost: 2021 - $715.8; 2020 - $684.1)	2,900,000		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		2,800,000		400,000
Equity Securities at Fair Value (Cost: 2021 - $715.8; 2020 - $684.1)	2,800,000		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	10,500,000		10,500,000		10,600,000
12 Months or Longer	1,700,000		1,700,000		3,100,000
Fixed maturities, allowance for credit losses	5,100,000	4,800,000	5,100,000	4,800,000	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	46,600,000		46,600,000
Variable Interest Entity, Funding Investment Commitment, Funding	0	$ 0	48,500,000	$ 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	91,800,000		91,800,000
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		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	11,500,000		11,500,000		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	$ 8,500,000		$ 8,500,000		$ 23,700,000
Percentage of unrealized loss position to amortized cost basis of available for sale security average	8.00%		8.00%		11.00%