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, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Impairment Losses	$ 0.5	$ (1.0)	$ (7.0)	$ (12.0)	$ (1.7)	$ (1.8)	$ (6.7)	$ (3.6)	$ (19.5)	$ (13.8)	$ (4.5)
Income Tax Benefit (Expense)	(23.1)	(29.8)	(33.6)	(13.7)	(32.2)	(32.5)	(26.4)	(39.4)	(100.2)	(130.5)	(10.7)
Net Income	$ 97.5	$ 122.3	$ 126.1	$ 64.0	$ 124.7	$ 129.0	$ 122.1	$ 155.3	409.9	531.1	190.1
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net Income									(42.1)	24.3	16.2
Having No Credit Losses Recognized in Consolidated Statements of Income | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net Realized Gains on Sales of Investments									36.3	41.9	26.4
Impairment Losses									(19.5)	(13.8)	(4.5)
Total Before Income Taxes									16.8	28.1	21.9
Income Tax Benefit (Expense)									(3.6)	(5.8)	(4.6)
Net Income									13.2	22.3	17.3
Net Unrecognized Postretirement Benefit Costs | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest and Other (Expenses) Income									(70.2)	3.0	(1.1)
Income Tax Benefit (Expense)									15.3	(0.7)	0.2
Net Income									(54.9)	2.3	(0.9)
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)	(0.4)	(0.3)
Income Tax Benefit (Expense)									0.0	0.1	0.1
Net Income									$ (0.4)	$ (0.3)	$ (0.2)