Schedule of Reclassifications 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 Adjustment out of Accumulated Other Comprehensive Income
Net Realized Investment Gain (Loss), Excluding Other-Than-Temporary Impairment Loss on Fixed Maturity Securities	$ (17.2)	$ 1.2	$ (19.1)	$ 33.4
Other-Than-Temporary Impairment Loss	(9.4)	0.0	(22.0)	0.0
Net Investment Income	612.1	611.3	1,844.8	1,859.5
Interest and Debt Expense	(38.1)	(38.4)	(113.9)	(129.0)
Other Expenses	(200.1)	(187.1)	(603.6)	(566.0)
Income Before Income Tax	298.9	317.0	921.7	996.6
Total Income Tax	95.1	97.9	280.7	312.3
Net Income	203.8	219.1	641.0	684.3
Reclassification out of Accumulated Other Comprehensive Income | Net Unrealized Gain on Securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Net Realized Investment Gain (Loss), Excluding Other-Than-Temporary Impairment Loss on Fixed Maturity Securities	10.2	5.4	12.4	2.6
Other-Than-Temporary Impairment Loss	(9.4)	0.0	(22.0)	0.0
Income Before Income Tax	0.8	5.4	(9.6)	2.6
Total Income Tax	0.3	1.7	(3.7)	0.6
Net Income	0.5	3.7	(5.9)	2.0
Reclassification out of Accumulated Other Comprehensive Income | Net Gain on Cash Flow Hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Income Before Income Tax	12.1	9.2	36.2	41.5
Total Income Tax	4.2	3.3	12.4	12.6
Net Income	7.9	5.9	23.8	28.9
Reclassification out of Accumulated Other Comprehensive Income | Net Gain on Cash Flow Hedges | Interest Rate Swaps
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Net Realized Investment Gain (Loss), Excluding Other-Than-Temporary Impairment Loss on Fixed Maturity Securities	0.3	(0.2)	0.6	2.3
Net Investment Income	12.8	12.0	38.3	35.9
Interest and Debt Expense	(0.5)	(0.4)	(1.4)	(1.3)
Reclassification out of Accumulated Other Comprehensive Income | Net Gain on Cash Flow Hedges | Foreign Exchange Contracts
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Net Realized Investment Gain (Loss), Excluding Other-Than-Temporary Impairment Loss on Fixed Maturity Securities	(0.2)	(1.1)	0.0	7.9
Net Investment Income	(0.3)	(1.1)	(1.3)	(3.3)
Reclassification out of Accumulated Other Comprehensive Income | Net Actuarial Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Other Expenses	(3.0)	(1.4)	(8.9)	(4.2)
Reclassification out of Accumulated Other Comprehensive Income | Prior Service Credit
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Other Expenses	0.0	0.4	0.1	1.2
Reclassification out of Accumulated Other Comprehensive Income | Unrecognized Pension and Postretirement Benefit Costs
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Income Before Income Tax	(3.0)	(1.0)	(8.8)	(3.0)
Total Income Tax	(1.1)	(0.3)	(3.1)	(1.0)
Net Income	$ (1.9)	$ (0.7)	$ (5.7)	$ (2.0)