Investments - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Net Investment Income [Line Items]
Investment income	$ 101.0	$ 83.1	$ 188.5	$ 167.1
Investment expenses	5.0	4.7	9.8	9.5
Net investment income	96.0	78.4	178.7	157.6
Fixed Maturities
Net Investment Income [Line Items]
Investment income	75.6	64.4	152.0	126.8
Equity Securities
Net Investment Income [Line Items]
Investment income	6.9	2.6	9.4	4.5
Total Alternative Investments
Net Investment Income [Line Items]
Investment income	7.7	6.6	6.7	17.6
Equity Method Limited Liability Investments
Net Investment Income [Line Items]
Investment income	2.6	1.4	(1.0)	8.5
Limited Liability Investments Included in Equity Securities
Net Investment Income [Line Items]
Investment income	5.1	5.2	7.7	9.1
Short-term Investments
Net Investment Income [Line Items]
Investment income	2.6	1.2	4.4	2.0
Loans to Policyholders
Net Investment Income [Line Items]
Investment income	5.7	5.5	11.0	11.0
Real Estate
Net Investment Income [Line Items]
Investment income	2.2	2.4	4.7	4.8
Investment expenses	2.3	2.6	4.7	4.9
Other
Net Investment Income [Line Items]
Investment income	0.3	0.4	0.3	0.4
Investment expenses	$ 2.7	$ 2.1	$ 5.1	$ 4.6