Schedule of Reclassifications Out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Net Realized Investment Gain (Loss), Excluding Other-Than-Temporary Impairment Loss on Fixed Maturity Securities	$ 1.1	$ (1.2)
Other-Than-Temporary Impairment Loss	0.0	(1.0)
Net Investment Income	594.7	602.3
Interest and Debt Expense	(42.1)	(40.2)
Other Expenses	(237.9)	(224.2)
Income Before Income Tax	353.3	341.1
Total Income Tax	72.4	67.6
Net Income	280.9	273.5
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	(8.7)	0.2
Other-Than-Temporary Impairment Loss	0.0	(1.0)
Income Before Income Tax	(8.7)	(0.8)
Total Income Tax	(1.8)	(0.2)
Net Income	(6.9)	(0.6)
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	20.0	14.7
Total Income Tax	4.2	3.0
Net Income	15.8	11.7
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	4.0	(0.1)
Net Investment Income	17.4	15.6
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	(0.8)	0.0
Net Investment Income	(0.1)	(0.3)
Reclassification out of Accumulated Other Comprehensive Income or Loss | Net Actuarial Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Other Expenses	(4.7)	(5.6)
Reclassification out of Accumulated Other Comprehensive Income or Loss | Prior Service Credit
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Other Expenses	0.1	0.1
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	(4.6)	(5.5)
Total Income Tax	(1.0)	(1.2)
Net Income	$ (3.6)	$ (4.3)