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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net Impairment Losses Recognized in Earnings	$ (1.7)	$ (1.8)	$ (6.7)	$ (3.6)	$ (2.2)	$ (1.8)	$ 0.0	$ (0.5)	$ (13.8)	$ (4.5)	$ (14.3)
Income Tax Benefit (Expense)	(32.2)	(32.5)	(26.4)	(39.4)	(1.1)	11.8	(8.0)	(13.4)	(130.5)	(10.7)	(41.2)
Net Income	$ 124.7	$ 129.0	$ 122.1	$ 155.3	$ 6.5	$ 92.2	$ 37.6	$ 53.8	531.1	190.1	120.9
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net Income									24.3	16.2	22.5
Accumulated Net Unrealized Investment Gain (Loss) | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net Realized Gains on Sales of Investments									41.9	26.4	49.6
Net Impairment Losses Recognized in Earnings									(13.8)	(4.5)	(14.3)
Total Before Income Taxes									28.1	21.9	35.3
Income Tax Benefit (Expense)									(5.8)	(4.6)	(12.3)
Net Income									22.3	17.3	23.0
Accumulated Defined Benefit Plans Adjustment | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest and Other Expenses (Income)									3.0	(1.1)	0.6
Income Tax Benefit (Expense)									(0.7)	0.2	(0.2)
Net Income									2.3	(0.9)	0.4
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest and Other Expenses (Income)									(0.4)	(0.3)	(1.3)
Income Tax Benefit (Expense)									0.1	0.1	0.4
Net Income									$ (0.3)	$ (0.2)	$ (0.9)