Income from Investments - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Net Investment Income [Line Items]
Investment income									$ 383.4	$ 359.9	$ 345.0
Investment expenses									19.1	19.0	17.8
Net investment income	$ 93.9	$ 91.7	$ 96.0	$ 82.7	$ 91.3	$ 92.0	$ 78.4	$ 79.2	364.3	340.9	327.2
Interest on Fixed Income Securities
Net Investment Income [Line Items]
Investment income									299.4	268.9	246.6
Dividends on Equity Securities Excluding Alternative Investments
Net Investment Income [Line Items]
Investment income									22.9	13.6	9.3
Total Alternative Investments
Net Investment Income [Line Items]
Investment income									19.0	37.4	54.7
Equity Method Limited Liability Investments
Net Investment Income [Line Items]
Investment income									1.0	11.0	24.8
Fair Value Option Investments
Net Investment Income [Line Items]
Investment income									0.0	0.0	1.3
Limited Liability Investments Included in Equity Securities
Net Investment Income [Line Items]
Investment income									18.0	26.4	28.6
Short-term Investments
Net Investment Income [Line Items]
Investment income									8.2	7.0	1.6
Loans to Policyholders
Net Investment Income [Line Items]
Investment income									22.6	22.5	21.6
Real Estate
Net Investment Income [Line Items]
Investment income									9.8	9.6	10.7
Investment expenses									9.6	9.7	10.5
Other
Net Investment Income [Line Items]
Investment income									1.5	0.9	0.5
Investment expenses									$ 9.5	$ 9.3	$ 7.3