Investments - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Net Investment Income [Line Items]
Investment income	$ 107.5	$ 109.8	$ 344.0	$ 344.2
Investment expenses	9.7	7.9	27.7	25.3
Net Investment Income	97.8	101.9	316.3	318.9
Fixed Maturities
Net Investment Income [Line Items]
Investment income	76.9	68.4	218.4	207.1
Equity Securities
Net Investment Income [Line Items]
Investment income	1.1	2.9	4.3	9.8
Total Alternative Investments
Net Investment Income [Line Items]
Investment income	8.2	21.5	62.9	80.2
Equity Method Limited Liability Investments
Net Investment Income [Line Items]
Investment income	(0.6)	12.0	28.0	50.9
Limited Liability Investments Included in Equity Securities
Net Investment Income [Line Items]
Investment income	8.8	9.5	34.9	29.3
Short-term Investments
Net Investment Income [Line Items]
Investment income	1.1	0.2	1.4	0.6
Loans to Policyholders
Net Investment Income [Line Items]
Investment income	5.5	5.4	16.3	16.3
Real Estate
Net Investment Income [Line Items]
Investment income	3.1	2.3	7.6	7.1
Investment expenses	2.3	2.6	5.8	6.8
Other Investment Expenses
Net Investment Income [Line Items]
Investment expenses	7.4	5.3	21.9	18.5
Universal Life-type Policyholder Account Balances
Net Investment Income [Line Items]
Investment income	1.7	1.9	5.1	4.8
Cash Surrender Value
Net Investment Income [Line Items]
Investment income	$ 9.9	$ 7.2	$ 28.0	$ 18.3