Investments - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Net Investment Income [Line Items]
Investment income	$ 122.5	$ 76.8	$ 234.4	$ 171.6
Investment expenses	8.6	9.0	17.4	18.2
Net Investment Income	113.9	67.8	217.0	153.4
Fixed Maturities
Net Investment Income [Line Items]
Investment income	69.7	74.3	138.7	145.3
Equity Securities
Net Investment Income [Line Items]
Investment income	4.8	3.6	6.9	7.9
Total Alternative Investments
Net Investment Income [Line Items]
Investment income	31.7	(11.6)	58.7	(6.0)
Equity Method Limited Liability Investments
Net Investment Income [Line Items]
Investment income	16.4	(12.7)	38.9	(10.9)
Limited Liability Investments Included in Equity Securities
Net Investment Income [Line Items]
Investment income	15.3	1.1	19.8	4.9
Short-term Investments
Net Investment Income [Line Items]
Investment income	(0.8)	0.3	0.4	1.9
Loans to Policyholders
Net Investment Income [Line Items]
Investment income	5.4	5.5	10.9	11.1
Real Estate
Net Investment Income [Line Items]
Investment income	2.4	2.3	4.8	4.8
Investment expenses	2.1	2.7	4.2	5.3
Other
Net Investment Income [Line Items]
Investment income	9.3	2.4	14.0	6.6
Investment expenses	$ 6.5	$ 6.3	$ 13.2	$ 12.9