Investments Investments - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Net Investment Income [Line Items]
Investment Income, Equity Method Limited Liability Investments	$ 11.1	$ 5.2	$ 21.9	$ 3.7
Investment Income, Fair Value Option Investments	0.5	(1.4)	1.0	(3.9)
Investment Income, Limited Liability Investments Included in Equity Securities	5.9	5.7	18.7	16.6
Investment Income, Alternative Investments	17.5	9.5	41.6	16.4
Investment Income, Real Estate	2.6	3.0	8.3	8.9
Gross Investment Income	90.0	82.2	257.8	232.0
Investment Income, Investment Expense	4.1	4.5	13.2	13.6
Net Investment Income	85.9	77.7	244.6	218.4
Fixed Maturities [Member]
Net Investment Income [Line Items]
Interest and Dividend Income, Operating	61.8	61.4	184.0	181.2
Equity Securities [Member]
Net Investment Income [Line Items]
Interest and Dividend Income, Operating	2.6	2.7	7.0	9.0
Short-term Investments [Member]
Net Investment Income [Line Items]
Interest and Dividend Income, Operating	0.5	0.2	1.0	0.4
Policy Loans [Member]
Net Investment Income [Line Items]
Interest and Dividend Income, Operating	4.9	5.4	15.7	16.1
Other Investment [Member]
Net Investment Income [Line Items]
Interest and Dividend Income, Operating	0.1	0.0	0.2	0.0
Real Estate Investment [Member]
Net Investment Income [Line Items]
Investment Income, Investment Expense	2.6	2.9	7.7	8.4
Other Investment [Member]
Net Investment Income [Line Items]
Investment Income, Investment Expense	$ 1.5	$ 1.6	$ 5.5	$ 5.2