Income from Investments - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net Investment Income [Line Items]
Investment income									$ 382.6	$ 383.4	$ 359.9
Investment expenses									34.4	19.1	19.0
Net investment income	$ 102.7	$ 92.1	$ 67.8	$ 85.6	$ 93.9	$ 91.7	$ 96.0	$ 82.7	348.2	364.3	340.9
Interest on Fixed Income Securities
Net Investment Income [Line Items]
Investment income									289.8	299.4	268.9
Dividends on Equity Securities Excluding Alternative Investments
Net Investment Income [Line Items]
Investment income									15.4	22.9	13.6
Total Alternative Investments
Net Investment Income [Line Items]
Investment income									27.0	19.0	37.4
Equity Method Limited Liability Investments
Net Investment Income [Line Items]
Investment income									4.9	1.0	11.0
Limited Liability Investments Included in Equity Securities
Net Investment Income [Line Items]
Investment income									22.1	18.0	26.4
Short-term Investments
Net Investment Income [Line Items]
Investment income									5.5	8.2	7.0
Loans to Policyholders
Net Investment Income [Line Items]
Investment income									22.1	22.6	22.5
Real Estate
Net Investment Income [Line Items]
Investment income									9.6	9.8	9.6
Investment expenses									8.8	9.6	9.7
Other
Net Investment Income [Line Items]
Investment income									13.2	1.5	0.9
Investment expenses									$ 25.6	$ 9.5	$ 9.3