Schedule of Reclassifications Out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Net Realized Investment Gain (Loss), Excluding Other-Than-Temporary Impairment Loss on Fixed Maturity Securities	$ (1.2)	$ 11.0
Other-Than-Temporary Impairment Loss	(1.0)	0.0
Net Investment Income	602.3	602.4
Interest and Debt Expense	(40.2)	(39.8)
Other Expenses	(224.2)	(228.8)
Income Before Income Tax	341.1	330.3
Total Income Tax	67.6	100.4
Net Income	273.5	229.9
Reclassification out of Accumulated Other Comprehensive Income or Loss | 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	0.2	3.4
Other-Than-Temporary Impairment Loss	(1.0)	0.0
Income Before Income Tax	(0.8)	3.4
Total Income Tax	(0.2)	1.1
Net Income	(0.6)	2.3
Reclassification out of Accumulated Other Comprehensive Income or Loss | Accumulated Net Gain (Loss) from Designated or Qualifying Hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Income Before Income Tax	14.7	12.3
Total Income Tax	3.0	4.3
Net Income	11.7	8.0
Reclassification out of Accumulated Other Comprehensive Income or Loss | Accumulated Net Gain (Loss) from Designated or Qualifying 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	(0.1)	0.0
Net Investment Income	15.6	14.3
Interest and Debt Expense	(0.5)	(0.5)
Reclassification out of Accumulated Other Comprehensive Income or Loss | Accumulated Net Gain (Loss) from Designated or Qualifying 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	(0.3)	(0.3)
Reclassification out of Accumulated Other Comprehensive Income or Loss | Net Actuarial Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Other Expenses	(5.6)	(5.1)
Reclassification out of Accumulated Other Comprehensive Income or Loss | Prior Service Credit
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Other Expenses	0.1	0.2
Reclassification out of Accumulated Other Comprehensive Income or Loss | Unrecognized Pension and Postretirement Benefit Costs
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Income Before Income Tax	(5.5)	(4.9)
Total Income Tax	(1.2)	(1.7)
Net Income	$ (4.3)	$ (3.2)