Other Comprehensive Income (Loss) and Accumulated Other Comprehensive Income - Reclassification out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Impairment Losses	$ (12.0)	$ (3.6)
Income from Continuing Operations before Income Taxes	77.7	194.7
Income Tax Benefit (Expense)	(13.7)	(39.4)
Net Income	64.0	155.3
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Net Income	2.4	10.4
Accumulated Net Unrealized Investment Gain (Loss) | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Net Realized Gains on Sales of Investments	15.9	16.1
Impairment Losses	(12.0)	(3.6)
Income from Continuing Operations before Income Taxes	3.9	12.5
Income Tax Benefit (Expense)	(0.8)	(2.6)
Net Income	3.1	9.9
Accumulated Defined Benefit Plans Adjustment | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Interest and Other Expenses	(0.7)	0.6
Income Tax Benefit (Expense)	0.1	(0.2)
Net Income	(0.6)	0.4
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Interest and Other Expenses	(0.1)	0.1
Income Tax Benefit (Expense)	0.0	0.0
Net Income	$ (0.1)	$ 0.1