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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Net Realized Gains on Sales of Investments	$ 3.6	$ 8.1	$ 10.0	$ 45.0
Net Impairment Losses Recognized in Earnings	(1.8)	(2.9)	(2.3)	(10.5)
Interest and Other Expenses	(61.7)	(18.2)	(116.4)	(59.1)
Total Before Income Taxes	80.5	68.3	193.0	116.9
Income Tax Expense	11.8	(20.5)	(9.6)	(32.9)
Net Income	92.2	47.7	183.6	84.0
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Net Income	4.9	2.1	5.2	17.3
Accumulated Net Unrealized Investment Gain (Loss) [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Net Realized Gains on Sales of Investments	8.3	6.5	10.0	38.2
Net Impairment Losses Recognized in Earnings	(1.8)	(2.9)	(2.3)	(10.5)
Total Before Income Taxes	6.5	3.6	7.7	27.7
Income Tax Expense	(1.3)	(1.3)	(1.6)	(9.7)
Net Income	5.2	2.3	6.1	18.0
Accumulated Defined Benefit Plans Adjustment [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Interest and Other Expenses	(0.2)	0.1	(0.8)	0.4
Income Tax Expense	0.0	(0.1)	0.1	(0.2)
Net Income	(0.2)	0.0	(0.7)	0.2
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Interest and Other Expenses	(0.2)	(0.2)	(0.3)	(1.3)
Income Tax Expense	0.1	0.0	0.1	0.4
Net Income	$ (0.1)	$ (0.2)	$ (0.2)	$ (0.9)