Investments - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Net Investment Income [Line Items]
Investment income	$ 120.7	$ 127.6	$ 233.5	$ 236.5
Investment expenses	14.4	9.1	25.4	18.0
Net Investment Income	106.3	118.5	208.1	218.5
Fixed Maturities
Net Investment Income [Line Items]
Investment income	87.1	72.8	173.3	141.5
Equity Securities
Net Investment Income [Line Items]
Investment income	1.2	1.7	2.2	3.2
Total Alternative Investments
Net Investment Income [Line Items]
Investment income	9.1	33.8	12.8	54.7
Equity Method Limited Liability Investments
Net Investment Income [Line Items]
Investment income	2.8	15.3	3.9	28.6
Limited Liability Investments Included in Equity Securities
Net Investment Income [Line Items]
Investment income	6.3	18.5	8.9	26.1
Short-term Investments
Net Investment Income [Line Items]
Investment income	3.6	0.2	5.9	0.3
Loans to Policyholders
Net Investment Income [Line Items]
Investment income	5.1	5.3	10.5	10.8
Real Estate
Net Investment Income [Line Items]
Investment income	1.9	2.3	4.3	4.5
Investment expenses	2.2	1.0	4.3	3.5
Other Investment Expenses
Net Investment Income [Line Items]
Investment expenses	12.2	8.1	21.1	14.5
Universal Life-type Policyholder Account Balances
Net Investment Income [Line Items]
Investment income	5.3	1.7	8.3	3.4
Cash Surrender Value
Net Investment Income [Line Items]
Investment income	$ 7.4	$ 9.8	$ 16.2	$ 18.1