Investments Investments - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Net Investment Income [Line Items]
Investment Income, Equity Method Limited Liability Investments	$ (3.6)	$ 7.1
Investment Income, Limited Liability Investments Included in Equity Securities	2.6	3.9
Investment Income, Alternative Investments	(1.0)	11.0
Investment Income, Real Estate	2.5	2.4
Gross Investment Income	87.5	84.0
Investment Income, Investment Expense	4.8	4.8
Net Investment Income	82.7	79.2
Fixed Maturities:
Net Investment Income [Line Items]
Interest and Dividend Income, Operating	76.4	62.4
Equity Securities [Member]
Net Investment Income [Line Items]
Interest and Dividend Income, Operating	2.5	1.9
Short-term Investments [Member]
Net Investment Income [Line Items]
Interest and Dividend Income, Operating	1.8	0.8
Policy Loans [Member]
Net Investment Income [Line Items]
Interest and Dividend Income, Operating	5.3	5.5
Real Estate Investment [Member]
Net Investment Income [Line Items]
Investment Income, Investment Expense	2.4	2.3
Other Investment [Member]
Net Investment Income [Line Items]
Investment Income, Investment Expense	$ 2.4	$ 2.5