Other Comprehensive Income (Loss) And Accumulated Other Comprehensive Income Reclassification from AOCI to Consolidated Statements of 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
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net Realized Gains (Losses) on Sales of Investments	$ 11.5	$ 8.1	$ 26.4	$ 10.5	$ 9.1	$ 11.6	$ 5.6	$ 6.8	$ 56.5	$ 33.1	$ 52.1
Net Impairment Losses Recognized in Earnings	(3.8)	(2.9)	(2.6)	(5.0)	(8.7)	(8.3)	(6.4)	(9.3)	(14.3)	(32.7)	(27.2)
Interest and Other Expenses	(21.5)	(18.2)	(21.4)	(19.5)	(25.3)	(22.0)	(20.7)	(22.3)	(80.6)	(90.3)	(107.6)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax									1.3	0.0	0.0
Income Tax Benefit (Expense)	(8.3)	(20.5)	(15.5)	3.1	(11.5)	14.9	1.5	4.3	(41.2)	9.2	(20.1)
Net Income	$ 36.9	$ 47.7	$ 36.6	$ (0.3)	$ 31.2	$ (16.3)	$ 4.0	$ (2.1)	120.9	16.8	85.7
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net Income									22.5	(4.1)	1.4
Accumulated Net Unrealized Investment Gain (Loss) [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net Realized Gains (Losses) on Sales of Investments									49.6	32.0	52.6
Net Impairment Losses Recognized in Earnings									(14.3)	(32.2)	(27.2)
Total Before Income Taxes									35.3	(0.2)	25.4
Income Tax Benefit (Expense)									(12.3)	0.1	(8.9)
Net Income									23.0	(0.1)	16.5
Accumulated Defined Benefit Plans Adjustment [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest and Other Expenses									0.6	(6.3)	(23.1)
Income Tax Benefit (Expense)									(0.2)	2.3	8.0
Net Income									0.4	(4.0)	(15.1)
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest and Other Expenses									(1.3)	0.0	0.0
Income Tax Benefit (Expense)									0.4	0.0	0.0
Net Income									$ (0.9)	$ 0.0	$ 0.0