Investments - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-Sale, Maturity, without Single Maturity Date, Fair Value	$ 1,664.2
Financing Receivable, Accrued Interest, before Allowance for Credit Loss	74.8	$ 77.0
Fixed maturities, allowance for credit losses	(7.7)	(8.2)	$ 7.4	$ 9.6
Equity securities without readily determinable fair value, impairment loss, cumulative amount	$ 7.1
Percentage of Equity Method Limited Liability Investments reported with a lag	2.90%
Percentage of Equity Method Limited Liability Investments reported with a One Month Lag	4.80%
Company-Owned Life Insurance	$ 515.7	513.5
Loans to Policyholders	280.5
Loans to Policyholders		281.2
Equity Securities
Debt Securities, Available-for-sale [Line Items]
Other than Temporary Impairment Losses, Investments	0.4
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	227.4
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	95.3
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	62.5
Other Non-Governmental Issuers
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-Sale, Maturity, without Single Maturity Date, Fair Value	1,279.1
Investments in Fixed Maturities
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, unrealized loss position, accumulated loss	808.1	730.3
12 Months or Longer	790.8	721.2
Other Receivables	6.0	0.9
Other liabilities	21.7	0.0
Equity Securities
Debt Securities, Available-for-sale [Line Items]
Other Receivables	0.1	0.1
Equity Securities | Fair Value, Measurements, Recurring
Debt Securities, Available-for-sale [Line Items]
Equity securities	230.1	225.8
Equity Securities, FV-NI, Current	230.1	225.8
Equity Securities | Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring
Debt Securities, Available-for-sale [Line Items]
Equity securities	9.6	8.5
Equity Securities, FV-NI, Current	9.6	8.5
Preferred Stocks | All Industries Other than Finance, Insurance and Real Estate [Member] | Fair Value, Measurements, Recurring
Debt Securities, Available-for-sale [Line Items]
Equity securities	10.0	9.9
Equity Securities, FV-NI, Current	10.0	9.9
Preferred Stocks | All Industries Other than Finance, Insurance and Real Estate [Member] | Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring
Debt Securities, Available-for-sale [Line Items]
Equity securities	0.0	0.0
Equity Securities, FV-NI, Current	0.0	0.0
Common Stock [Member] | All Industries Other than Finance, Insurance and Real Estate [Member] | Fair Value, Measurements, Recurring
Debt Securities, Available-for-sale [Line Items]
Equity securities	0.8	0.6
Equity Securities, FV-NI, Current	0.8	0.6
Common Stock [Member] | All Industries Other than Finance, Insurance and Real Estate [Member] | Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring
Debt Securities, Available-for-sale [Line Items]
Equity securities	0.3	0.2
Equity Securities, FV-NI, Current	0.3	0.2
Corporate Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, unrealized loss position, accumulated loss	448.0	383.8
12 Months or Longer	436.5	378.3
Fixed maturities, allowance for credit losses	(7.2)	(7.7)	$ 6.6	$ 8.9
Equity Method Investments
Debt Securities, Available-for-sale [Line Items]
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	17.3
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	84.5
External Credit Rating, Investment Grade [Member] | Investments in Fixed Maturities
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, unrealized loss position, accumulated loss	788.7	704.8
Non-investment-grade | Investments in Fixed Maturities
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, unrealized loss position, accumulated loss	$ 19.4	$ 25.5
Percentage of unrealized loss position to amortized cost basis of available for sale security average	6.40%	8.80%