Income From Investments - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investment Income:
Equity Method Limited Liability Investments									$ 24.8	$ 7.5	$ 19.0
Fair Value Option Investments									1.3	(1.9)	0.2
Investment Income, Limited Liability Investments Included in Equity Securities									28.6	22.0	17.6
Investment Income, Alternative Investments									54.7	27.6	36.8
Real Estate									10.7	11.8	11.9
Total Investment Income									345.0	316.3	321.2
Investment Expenses:
Other Investment Expenses									17.8	18.0	18.6
Net Investment Income	$ 82.6	$ 85.9	$ 77.1	$ 81.6	$ 79.9	$ 77.7	$ 73.7	$ 67.0	327.2	298.3	302.6
Accrued Investment Income Receivable	$ 72.8				$ 70.8				72.8	70.8
Real Estate Investment [Member]
Investment Expenses:
Other Investment Expenses									10.5	11.0	11.3
Other Investment [Member]
Investment Expenses:
Other Investment Expenses									7.3	7.0	7.3
Fixed Maturities [Member]
Investment Income:
Interest and Dividends on Fixed Maturities									246.6	242.7	236.2
Equity Securities [Member]
Investment Income:
Interest and Dividends on Fixed Maturities									9.3	11.8	14.8
Short-term Investments [Member]
Investment Income:
Interest and Dividends on Fixed Maturities									1.6	0.5	0.4
Policy Loans [Member]
Investment Income:
Interest and Dividends on Fixed Maturities									21.6	21.6	21.1
Other Investment [Member]
Investment Income:
Interest and Dividends on Fixed Maturities									$ 0.5	$ 0.3	$ 0.0