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, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Net Realized Gains on Sales of Investments	$ 5.3	$ 8.0	$ 42.7	$ 18.1
Net Impairment Losses Recognized in Earnings	(3.3)	(3.8)	(12.5)	(8.7)
Total Before Income Taxes	49.8	4.0	101.9	67.1
Income Tax Expense	(11.8)	0.8	(23.0)	(17.8)
Interest and Other Expenses	(25.7)	(22.1)	(82.0)	(67.3)
Total Reclassification from AOCI to Net Income	37.9	4.7	81.1	49.1
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Total Reclassification from AOCI to Net Income	(2.3)	1.5	8.5	1.4
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	5.6	8.1	43.1	16.7
Net Impairment Losses Recognized in Earnings	(3.3)	(3.8)	(12.5)	(8.7)
Total Before Income Taxes	2.3	4.3	30.6	8.0
Income Tax Expense	(0.9)	(1.5)	(10.8)	(2.8)
Total Reclassification from AOCI to Net Income	1.4	2.8	19.8	5.2
Accumulated Defined Benefit Plans Adjustment [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Income Tax Expense	2.1	0.7	6.0	2.1
Interest and Other Expenses	(5.8)	(2.0)	(17.3)	(5.9)
Total Reclassification from AOCI to Net Income	$ (3.7)	$ (1.3)	$ (11.3)	$ (3.8)