Income From Investments - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Investment Income:
Equity Method Limited Liability Investments									$ 11.0	$ 24.8	$ 7.5
Fair Value Option Investments									0.0	1.3	(1.9)
Investment Income, Limited Liability Investments Included in Equity Securities									26.4	28.6	22.0
Investment Income, Alternative Investments									37.4	54.7	27.6
Real Estate									9.6	10.7	11.8
Total Investment Income									359.9	345.0	316.3
Investment Expenses:
Other Investment Expenses									19.0	17.8	18.0
Net Investment Income	$ 91.3	$ 92.0	$ 78.4	$ 79.2	$ 82.6	$ 85.9	$ 77.1	$ 81.6	340.9	327.2	298.3
Accrued Investment Income Receivable	$ 77.9				$ 72.8				77.9	72.8
Real Estate Investment [Member]
Investment Expenses:
Other Investment Expenses									9.7	10.5	11.0
Other Investment [Member]
Investment Expenses:
Other Investment Expenses									9.3	7.3	7.0
Fixed Maturities [Member]
Investment Income:
Interest and Dividends on Fixed Maturities									268.9	246.6	242.7
Equity Securities [Member]
Investment Income:
Interest and Dividends on Fixed Maturities									13.6	9.3	11.8
Short-term Investments [Member]
Investment Income:
Interest and Dividends on Fixed Maturities									7.0	1.6	0.5
Policy Loans [Member]
Investment Income:
Interest and Dividends on Fixed Maturities									22.5	21.6	21.6
Other Investment [Member]
Investment Income:
Interest and Dividends on Fixed Maturities									$ 0.9	$ 0.5	$ 0.3