Other Comprehensive Income (Loss) and Accumulated Other Comprehensive Income - Reclassification out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Net Impairment Losses Recognized in Earnings	$ (6.7)	$ 0.0	$ (10.3)	$ (0.5)
Income from Continuing Operations before Income Taxes	148.5	45.5	343.2	112.5
Income Tax Expense/Benefit	(26.4)	(8.0)	(65.8)	(21.4)
Net Income	122.1	37.6	277.4	91.4
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Net Income	11.6	(1.0)	22.0	0.3
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	21.3	(0.7)	37.4	1.7
Net Impairment Losses Recognized in Earnings	(6.7)	0.0	(10.3)	(0.5)
Income from Continuing Operations before Income Taxes	14.6	(0.7)	27.1	1.2
Income Tax Expense/Benefit	(3.1)	0.1	(5.7)	(0.3)
Net Income	11.5	(0.6)	21.4	0.9
Accumulated Defined Benefit Plans Adjustment | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Income Tax Expense/Benefit	0.1	0.0	(0.1)	0.1
Net Income	0.1	(0.3)	0.5	(0.5)
Interest and Other Expenses	0.0	(0.3)	0.6	(0.6)
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 Expense/Benefit	0.0	(0.1)	0.0	0.0
Net Income	0.0	(0.1)	0.1	(0.1)
Interest and Other Expenses	$ 0.0	$ 0.0	$ 0.1	$ (0.1)