Investments Investments - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2017
Net Investment Income [Line Items]
Investment Income, Equity Method Limited Liability Investments	$ (0.4)	$ 11.1		$ 8.1	$ 21.9
Investment Income, Fair Value Option Investments	0.0	0.5		0.0	1.0
Investment Income, Limited Liability Investments Included in Equity Securities	13.7	5.9		22.8	18.7
Investment Income, Alternative Investments	13.3	17.5		30.9	41.6
Investment Income, Real Estate	2.4	2.6		7.2	8.3
Gross Investment Income	96.8	90.0		263.9	257.8
Investment Income, Investment Expense	4.8	4.1		14.3	13.2
Net Investment Income	92.0	85.9		249.6	244.6
Fixed Maturities:
Net Investment Income [Line Items]
Interest and Dividend Income, Operating	70.6	61.8		197.4	184.0
Equity Securities [Member]
Net Investment Income [Line Items]
Interest and Dividend Income, Operating	2.5	2.6		7.0	7.0
Short-term Investments [Member]
Net Investment Income [Line Items]
Interest and Dividend Income, Operating	2.3	0.5		4.3	1.0
Policy Loans [Member]
Net Investment Income [Line Items]
Interest and Dividend Income, Operating	5.5	4.9		16.5	15.7
Other Investment [Member]
Net Investment Income [Line Items]
Interest and Dividend Income, Operating	0.2	0.1	$ 0.2	0.6
Real Estate Investment [Member]
Net Investment Income [Line Items]
Investment Income, Investment Expense	2.6	2.6		7.5	7.7
Other Investment [Member]
Net Investment Income [Line Items]
Investment Income, Investment Expense	$ 2.2	$ 1.5		$ 6.8	$ 5.5