Other Comprehensive Income (Loss) and Accumulated Other Comprehensive Income - Reclassification out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Net Impairment Losses Recognized in Earnings	$ (1.8)	$ (1.8)	$ (12.1)	$ (2.3)
Income from Continuing Operations before Income Taxes	161.5	80.5	504.7	193.0
Income Tax Benefit (Expense)	(32.5)	11.8	(98.3)	(9.6)
Net Income	129.0	92.2	406.4	183.6
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Net Income	(0.3)	4.9	21.7	5.2
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	1.7	8.3	39.1	10.0
Net Impairment Losses Recognized in Earnings	(1.9)	(1.8)	(12.2)	(2.3)
Income from Continuing Operations before Income Taxes	(0.2)	6.5	26.9	7.7
Income Tax Benefit (Expense)	0.0	(1.3)	(5.7)	(1.6)
Net Income	(0.2)	5.2	21.2	6.1
Accumulated Defined Benefit Plans Adjustment | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Income Tax Benefit (Expense)	0.0	0.0	(0.1)	0.1
Net Income	0.0	(0.2)	0.5	(0.7)
Interest and Other Expenses	0.0	(0.2)	0.6	(0.8)
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]
Income Tax Benefit (Expense)	0.0	0.1	0.0	0.1
Net Income	(0.1)	(0.1)	0.0	(0.2)
Interest and Other Expenses	$ (0.1)	$ (0.2)	$ 0.0	$ (0.3)