Investments - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Net Investment Income [Line Items]
Investment income	$ 109.8	$ 98.9	$ 344.2	$ 270.5
Investment expenses	7.9	6.8	25.3	25.0
Net Investment Income	101.9	92.1	318.9	245.5
Fixed Maturities
Net Investment Income [Line Items]
Investment income	68.4	72.7	207.1	218.0
Equity Securities
Net Investment Income [Line Items]
Investment income	2.9	2.8	9.8	10.7
Total Alternative Investments
Net Investment Income [Line Items]
Investment income	21.5	10.6	80.2	4.6
Equity Method Limited Liability Investments
Net Investment Income [Line Items]
Investment income	12.0	8.2	50.9	(2.7)
Limited Liability Investments Included in Equity Securities
Net Investment Income [Line Items]
Investment income	9.5	2.4	29.3	7.3
Short-term Investments
Net Investment Income [Line Items]
Investment income	0.2	2.3	0.6	4.2
Loans to Policyholders
Net Investment Income [Line Items]
Investment income	5.4	5.5	16.3	16.6
Real Estate
Net Investment Income [Line Items]
Investment income	2.3	2.3	7.1	7.1
Investment expenses	2.6	1.2	6.8	6.5
Other
Net Investment Income [Line Items]
Investment income	9.1	2.7	23.1	9.3
Investment expenses	$ 5.3	$ 5.6	$ 18.5	$ 18.5