Investments Investments - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2018	Jun. 30, 2017
Net Investment Income [Line Items]
Investment Income, Equity Method Limited Liability Investments	$ 1.4		$ 4.0		$ 8.5	$ 10.8
Investment Income, Fair Value Option Investments	0.0		(0.6)		0.0	0.5
Investment Income, Limited Liability Investments Included in Equity Securities	5.2		5.4		9.1	12.8
Investment Income, Alternative Investments	6.6		8.8		17.6	24.1
Investment Income, Real Estate	2.4		2.8		4.8	5.7
Gross Investment Income	83.1		81.6		167.1	167.8
Investment Income, Investment Expense	4.7		4.5		9.5	9.1
Net Investment Income	78.4		77.1		157.6	158.7
Fixed Maturities:
Net Investment Income [Line Items]
Interest and Dividend Income, Operating	64.4		62.1		126.8	122.2
Equity Securities [Member]
Net Investment Income [Line Items]
Interest and Dividend Income, Operating	2.6		2.3		4.5	4.4
Short-term Investments [Member]
Net Investment Income [Line Items]
Interest and Dividend Income, Operating	1.2		0.2		2.0	0.5
Policy Loans [Member]
Net Investment Income [Line Items]
Interest and Dividend Income, Operating	5.5		5.3		11.0	10.8
Other Investment [Member]
Net Investment Income [Line Items]
Interest and Dividend Income, Operating	0.4	$ 0.1		$ 0.1	0.4
Real Estate Investment [Member]
Net Investment Income [Line Items]
Investment Income, Investment Expense	2.6		2.5		4.9	5.1
Other Investment [Member]
Net Investment Income [Line Items]
Investment Income, Investment Expense	$ 2.1		$ 2.0		$ 4.6	$ 4.0