Investments - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Net Investment Income [Line Items]
Investment income	$ 76.8	$ 101.0	$ 171.6	$ 188.5
Investment expenses	9.0	5.0	18.2	9.8
Net investment income	67.8	96.0	153.4	178.7
Fixed Maturities
Net Investment Income [Line Items]
Investment income	74.3	75.6	145.3	152.0
Equity Securities
Net Investment Income [Line Items]
Investment income	3.6	6.9	7.9	9.4
Total Alternative Investments
Net Investment Income [Line Items]
Investment income	(11.6)	7.7	(6.0)	6.7
Equity Method Limited Liability Investments
Net Investment Income [Line Items]
Investment income	(12.7)	2.6	(10.9)	(1.0)
Limited Liability Investments Included in Equity Securities
Net Investment Income [Line Items]
Investment income	1.1	5.1	4.9	7.7
Short-term Investments
Net Investment Income [Line Items]
Investment income	0.3	2.6	1.9	4.4
Loans to Policyholders
Net Investment Income [Line Items]
Investment income	5.5	5.7	11.1	11.0
Real Estate
Net Investment Income [Line Items]
Investment income	2.3	2.2	4.8	4.7
Investment expenses	2.7	2.3	5.3	4.7
Other
Net Investment Income [Line Items]
Investment income	2.4	0.3	6.6	0.3
Investment expenses	$ 6.3	$ 2.7	$ 12.9	$ 5.1