Other Comprehensive Income and Accumulated Other Comprehensive Income - Reclassification out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Net Realized Gains on Sales of Investments	$ 26.4	$ 5.6	$ 36.9	$ 12.4
Net Impairment Losses Recognized in Earnings	(2.6)	(6.4)	(7.6)	(15.7)
Interest and Other Expenses	(21.4)	(20.7)	(40.9)	(43.0)
Total Before Income Taxes	52.1	2.6	48.6	(3.9)
Income Tax Benefit (Expense)	(15.5)	1.5	(12.4)	5.8
Net Income	36.6	4.0	36.3	1.9
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Net Income	11.8	(2.9)	15.2	(6.1)
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	21.6	4.7	31.7	11.5
Net Impairment Losses Recognized in Earnings	(2.6)	(6.4)	(7.6)	(15.7)
Total Before Income Taxes	19.0	(1.7)	24.1	(4.2)
Income Tax Benefit (Expense)	(6.6)	0.6	(8.4)	1.5
Net Income	12.4	(1.1)	15.7	(2.7)
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	(2.7)	0.3	(5.3)
Income Tax Benefit (Expense)	(0.1)	0.9	(0.1)	1.9
Net Income	0.1	(1.8)	0.2	(3.4)
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	(1.1)	0.0	(1.1)	0.0
Income Tax Benefit (Expense)	0.4	0.0	0.4	0.0
Net Income	$ (0.7)	$ 0.0	$ (0.7)	$ 0.0