Income From Investments - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Investment Income:
Equity Method Limited Liability Investments									$ 7.5	$ 19.0	$ 9.0
Fair Value Option Investments									(1.9)	0.2	(0.7)
Investment Income, Limited Liability Investments Included in Equity Securities									22.0	17.6	40.7
Investment Income, Alternative Investments									27.6	36.8	49.0
Real Estate									11.8	11.9	12.1
Total Investment Income									316.3	321.2	328.9
Investment Expenses:
Other Investment Expenses									18.0	18.6	19.8
Net Investment Income	$ 79.9	$ 77.7	$ 73.7	$ 67.0	$ 79.4	$ 75.9	$ 76.7	$ 70.6	298.3	302.6	309.1
Accrued Investment Income Receivable	$ 70.8				$ 68.5				70.8	68.5
Real Estate Investment [Member]
Investment Expenses:
Other Investment Expenses									11.0	11.3	11.3
Other Investment [Member]
Investment Expenses:
Other Investment Expenses									7.0	7.3	8.5
Fixed Maturities [Member]
Investment Income:
Interest and Dividends on Fixed Maturities									242.7	236.2	227.4
Equity Securities [Member]
Investment Income:
Interest and Dividends on Fixed Maturities									11.8	14.8	19.2
Short-term Investments [Member]
Investment Income:
Interest and Dividends on Fixed Maturities									0.5	0.4	0.6
Policy Loans [Member]
Investment Income:
Interest and Dividends on Fixed Maturities									21.6	21.1	20.5
Other Investment [Member]
Investment Income:
Interest and Dividends on Fixed Maturities									$ 0.3	$ 0.0	$ 0.1