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, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Net Realized Gains on Sales of Investments	$ 34.0	$ 3.5	$ 37.4	$ 10.1
Net Impairment Losses Recognized in Earnings	(2.2)	(4.1)	(9.2)	(4.9)
Total Before Income Taxes	34.3	11.2	52.1	63.1
Interest and Other Expenses	(26.6)	(22.5)	(56.3)	(45.2)
Income Tax Benefit (Expense)	(6.9)	(1.9)	(11.2)	(18.6)
Total Reclassification from AOCI to Net Income	29.7	9.3	43.2	44.4
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Total Reclassification from AOCI to Net Income	16.8	(1.7)	10.8	(0.1)
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	34.1	3.4	37.5	8.6
Net Impairment Losses Recognized in Earnings	(2.2)	(4.1)	(9.2)	(4.9)
Total Before Income Taxes	31.9	(0.7)	28.3	3.7
Income Tax Benefit (Expense)	(11.2)	0.2	(9.9)	(1.3)
Total Reclassification from AOCI to Net Income	20.7	(0.5)	18.4	2.4
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	(6.1)	(1.9)	(11.5)	(3.9)
Income Tax Benefit (Expense)	2.2	0.7	3.9	1.4
Total Reclassification from AOCI to Net Income	$ (3.9)	$ (1.2)	$ (7.6)	$ (2.5)