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, 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
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net Realized Gains (Losses) on Sales of Investments									$ 26.4	$ 56.5	$ 33.1
Net Impairment Losses Recognized in Earnings	$ (2.2)	$ (1.8)	$ 0.0	$ (0.5)	$ (3.8)	$ (2.9)	$ (2.6)	$ (5.0)	(4.5)	(14.3)	(32.7)
Interest and Other Expenses	(42.6)	(61.7)	(25.7)	(29.0)	(21.5)	(18.2)	(21.4)	(19.5)	(159.0)	(80.6)	(90.3)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax									0.3	1.3	0.0
Income Tax Benefit (Expense)	(1.1)	11.8	(8.0)	(13.4)	(8.3)	(20.5)	(15.5)	3.1	(10.7)	(41.2)	9.2
Net Income	$ 6.5	$ 92.2	$ 37.6	$ 53.8	$ 36.9	$ 47.7	$ 36.6	$ (0.3)	190.1	120.9	16.8
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net Income									16.2	22.5	(4.1)
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									26.4	49.6	32.0
Net Impairment Losses Recognized in Earnings									(4.5)	(14.3)	(32.2)
Total Before Income Taxes									21.9	35.3	(0.2)
Income Tax Benefit (Expense)									(4.6)	(12.3)	0.1
Net Income									17.3	23.0	(0.1)
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									(1.1)	0.6	(6.3)
Income Tax Benefit (Expense)									0.2	(0.2)	2.3
Net Income									(0.9)	0.4	(4.0)
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									(0.3)	(1.3)	0.0
Income Tax Benefit (Expense)									0.1	0.4	0.0
Net Income									$ (0.2)	$ (0.9)	$ 0.0