Investments - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Net Investment Income [Line Items]
Investment income	$ 119.0	$ 107.5	$ 352.5	$ 344.0
Investment expenses	12.0	9.7	37.4	27.7
Net Investment Income	107.0	97.8	315.1	316.3
Fixed Maturities
Net Investment Income [Line Items]
Investment income	86.5	76.9	259.8	218.4
Equity Securities
Net Investment Income [Line Items]
Investment income	1.2	1.1	3.4	4.3
Total Alternative Investments
Net Investment Income [Line Items]
Investment income	9.7	8.2	22.5	62.9
Equity Method Limited Liability Investments
Net Investment Income [Line Items]
Investment income	4.3	(0.6)	8.2	28.0
Limited Liability Investments Included in Equity Securities
Net Investment Income [Line Items]
Investment income	5.4	8.8	14.3	34.9
Short-term Investments
Net Investment Income [Line Items]
Investment income	6.0	1.1	11.9	1.4
Loans to Policyholders
Net Investment Income [Line Items]
Investment income	5.1	5.5	15.6	16.3
Real Estate
Net Investment Income [Line Items]
Investment income	2.3	3.1	6.6	7.6
Investment expenses	1.7	2.3	6.0	5.8
Other Investment Expenses
Net Investment Income [Line Items]
Investment expenses	10.3	7.4	31.4	21.9
Universal Life-type Policyholder Account Balances
Net Investment Income [Line Items]
Investment income	1.8	1.7	10.1	5.1
Cash Surrender Value
Net Investment Income [Line Items]
Investment income	$ 6.4	$ 9.9	$ 22.6	$ 28.0