Investments - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Net Investment Income [Line Items]
Investment income	$ 98.9	$ 96.9	$ 270.5	$ 285.4
Investment expenses	6.8	5.2	25.0	15.0
Net investment income	92.1	91.7	245.5	270.4
Fixed Maturities
Net Investment Income [Line Items]
Investment income	72.7	73.5	218.0	225.5
Equity Securities
Net Investment Income [Line Items]
Investment income	2.8	5.2	10.7	14.6
Total Alternative Investments
Net Investment Income [Line Items]
Investment income	10.6	6.8	4.6	13.5
Equity Method Limited Liability Investments
Net Investment Income [Line Items]
Investment income	8.2	1.6	(2.7)	0.6
Limited Liability Investments Included in Equity Securities
Net Investment Income [Line Items]
Investment income	2.4	5.2	7.3	12.9
Short-term Investments
Net Investment Income [Line Items]
Investment income	2.3	2.5	4.2	6.9
Loans to Policyholders
Net Investment Income [Line Items]
Investment income	5.5	5.9	16.6	16.9
Real Estate
Net Investment Income [Line Items]
Investment income	2.3	2.4	7.1	7.1
Investment expenses	1.2	2.5	6.5	7.2
Other
Net Investment Income [Line Items]
Investment income	2.7	0.6	9.3	0.9
Investment expenses	$ 5.6	$ 2.7	$ 18.5	$ 7.8