Investments - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Net Investment Income [Line Items]
Investment income	$ 94.8	$ 87.5
Investment expenses	9.2	4.8
Net investment income	85.6	82.7
Fixed Maturities
Net Investment Income [Line Items]
Investment income	71.0	76.4
Equity Securities
Net Investment Income [Line Items]
Investment income	4.3	2.5
Total Alternative Investments
Net Investment Income [Line Items]
Investment income	5.6	(1.0)
Equity Method Limited Liability Investments
Net Investment Income [Line Items]
Investment income	1.8	(3.6)
Limited Liability Investments Included in Equity Securities
Net Investment Income [Line Items]
Investment income	3.8	2.6
Short-term Investments
Net Investment Income [Line Items]
Investment income	1.6	1.8
Loans to Policyholders
Net Investment Income [Line Items]
Investment income	5.6	5.3
Real Estate
Net Investment Income [Line Items]
Investment income	2.5	2.5
Investment expenses	2.6	2.4
Other
Net Investment Income [Line Items]
Investment income	4.2	0.0
Investment expenses	$ 6.6	$ 2.4