Investments - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-Sale, Maturity, without Single Maturity Date, Fair Value	$ 1,637.0		$ 1,637.0
Fixed maturities, allowance for credit losses	(8.6)	$ 7.5	(8.6)	$ 7.5	$ (9.6)	$ 7.5
Equity securities	233.4		233.4		243.2
Equity securities without readily determinable fair value, impairment loss, cumulative amount	7.5		7.5
Other Receivables	223.4		223.4		262.6
Equity Securities, FV-NI, Current	$ 233.4		$ 233.4		243.2
Percentage of Equity Method Limited Liability Investments reported with a lag	2.50%		2.50%
Percentage of Equity Method Limited Liability Investments reported with a One Month Lag	5.50%		5.50%
Company-Owned Life Insurance	$ 506.9		$ 506.9		586.5
Loans to Policyholders	281.8		281.8
Loans to Policyholders	281.8		281.8		283.4
Government National Mortgage Association Certificates and Obligations (GNMA)
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-Sale, Maturity, without Single Maturity Date, Fair Value	231.3		231.3
Federal National Mortgage Association Certificates and Obligations (FNMA)
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-Sale, Maturity, without Single Maturity Date, Fair Value	85.9		85.9
Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC)
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-Sale, Maturity, without Single Maturity Date, Fair Value	59.2		59.2
Other Non-Governmental Issuers
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-Sale, Maturity, without Single Maturity Date, Fair Value	1,260.6		1,260.6
Investments in Fixed Maturities
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, unrealized loss position, accumulated loss	1,149.0		1,149.0		936.8
12 Months or Longer	1,068.1		1,068.1		308.9
Other Receivables	0.9		0.9		5.8
Other liabilities	1.0		1.0		25.9
Equity Securities
Debt Securities, Available-for-sale [Line Items]
Other Receivables	20.7		20.7		0.0
Equity Securities | Fair Value, Measurements, Recurring
Debt Securities, Available-for-sale [Line Items]
Equity securities	233.4		233.4		243.2
Equity Securities, FV-NI, Current	233.4		233.4		243.2
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	7.9		7.9		13.4
Equity Securities, FV-NI, Current	7.9		7.9		13.4
Preferred Stocks | Other Industries | Fair Value, Measurements, Recurring
Debt Securities, Available-for-sale [Line Items]
Equity securities	11.7		11.7		10.8
Equity Securities, FV-NI, Current	11.7		11.7		10.8
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		0.0		0.0
Equity Securities, FV-NI, Current	0.0		0.0		0.0
Common Stock | Other Industries | Fair Value, Measurements, Recurring
Debt Securities, Available-for-sale [Line Items]
Equity securities	0.6		0.6		1.2
Equity Securities, FV-NI, Current	0.6		0.6		1.2
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	0.2		0.2		0.3
Equity Securities, FV-NI, Current	0.2		0.2		0.3
Corporate Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, unrealized loss position, accumulated loss	651.6		651.6		499.7
12 Months or Longer	597.5		597.5		125.8
Fixed maturities, allowance for credit losses	(8.1)	7.1	(8.1)	7.1	(8.9)	$ 7.5
Equity Method Investments
Debt Securities, Available-for-sale [Line Items]
Other Receivables	2.1		2.1		35.2
Other liabilities	0.0		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	17.2		17.2		16.3
Variable Interest Entity, Funding Investment Commitment, Funding	0.0	$ 0.0	0.0	$ 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	89.4		89.4
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	0.0		0.0		0.0
Investment-grade | Investments in Fixed Maturities
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, unrealized loss position, accumulated loss	1,120.1		1,120.1		904.0
Non-investment-grade | Investments in Fixed Maturities
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, unrealized loss position, accumulated loss	$ 28.9		$ 28.9		$ 32.8
Percentage of unrealized loss position to amortized cost basis of available for sale security average	9.40%		9.40%		11.00%