Schedule of Reclassifications Out of Accumulated Other Comprehensive Income (Details) - Reclassification out of Accumulated Other Comprehensive Income - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 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	$ 8.9	$ 25.9	$ (1.9)	$ 32.2
Other-Than-Temporary Impairment Loss	(8.1)	0.0	(12.6)	0.0
Net Investment Income	630.7	632.4	1,232.7	1,248.2
Interest and Debt Expense	(38.0)	(52.7)	(75.8)	(90.6)
Other Expenses	(210.1)	(190.1)	(403.5)	(378.9)
Income Before Income Tax	319.8	349.8	622.8	679.6
Total Income Tax	95.5	110.4	185.6	214.4
Net Income	224.3	239.4	437.2	465.2
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	7.0	0.9	2.2	(2.8)
Other-Than-Temporary Impairment Loss	(8.1)	0.0	(12.6)	0.0
Income Before Income Tax	(1.1)	0.9	(10.4)	(2.8)
Total Income Tax	(0.7)	0.2	(4.0)	(1.1)
Net Income	(0.4)	0.7	(6.4)	(1.7)
Net Gain on Cash Flow Hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Income Before Income Tax	13.8	21.7	24.1	32.3
Total Income Tax	4.6	5.5	8.2	9.3
Net Income	9.2	16.2	15.9	23.0
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	0.0	0.3	2.5
Net Investment Income	13.0	12.2	25.5	23.9
Interest and Debt Expense	(0.4)	(0.5)	(0.9)	(0.9)
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	1.4	11.1	0.2	9.0
Net Investment Income	(0.5)	(1.1)	(1.0)	(2.2)
Net Actuarial Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Other Expenses	(2.9)	(1.4)	(5.9)	(2.8)
Prior Service Credit
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Other Expenses	0.1	0.4	0.1	0.8
Unrecognized Pension and Postretirement Benefit Costs
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Income Before Income Tax	(2.8)	(1.0)	(5.8)	(2.0)
Total Income Tax	(1.0)	(0.4)	(2.0)	(0.7)
Net Income	$ (1.8)	$ (0.6)	$ (3.8)	$ (1.3)