Schedule of Reclassifications Out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Net Realized Investment Gain (Loss), Excluding Other-Than-Temporary Impairment Loss on Fixed Maturity Securities	$ 0.6	$ (10.8)
Other-Than-Temporary Impairment Loss	(21.1)	(4.5)
Net Investment Income	606.4	602.0
Interest and Debt Expense	(38.6)	(37.8)
Other Expenses	(205.6)	(193.4)
Income Before Income Tax	304.2	303.0
Total Income Tax	93.6	90.1
Net Income	210.6	212.9
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	5.6	(4.8)
Other-Than-Temporary Impairment Loss	(21.1)	(4.5)
Income Before Income Tax	(15.5)	(9.3)
Total Income Tax	(5.2)	(3.3)
Net Income	(10.3)	(6.0)
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	15.8	10.3
Total Income Tax	5.5	3.6
Net Income	10.3	6.7
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	3.2	0.0
Net Investment Income	13.2	12.5
Interest and Debt Expense	(0.5)	(0.5)
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	0.0	(1.2)
Net Investment Income	(0.1)	(0.5)
Reclassification out of Accumulated Other Comprehensive Income | Net Actuarial Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Other Expenses	(4.1)	(3.0)
Reclassification out of Accumulated Other Comprehensive Income | Prior Service Credit
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Other Expenses	0.1	0.0
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	(4.0)	(3.0)
Total Income Tax	(1.4)	(1.0)
Net Income	$ (2.6)	$ (2.0)