Schedule of Reclassifications Out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Other Net Realized Investment Gain (Loss)									$ 54.7	$ (11.4)	$ 29.6
Other-Than-Temporary Impairment Loss									(30.5)	(32.4)	(13.5)
Net Investment Income	$ 617.9	$ 611.4	$ 623.3	$ 606.4	$ 636.4	$ 612.1	$ 630.7	$ 602.0	2,459.0	2,481.2	2,492.2
Interest and Debt Expense									(166.0)	(152.8)	(167.5)
Other Expenses									(831.6)	(813.4)	(831.2)
Income Before Income Tax	371.2	332.1	340.2	304.2	316.6	298.9	319.8	303.0	1,347.7	1,238.3	542.0
Income Tax Expense (Benefit)									416.3	371.2	139.9
Net Income	$ 248.0	$ 236.0	$ 236.8	$ 210.6	$ 226.1	$ 203.8	$ 224.3	$ 212.9	931.4	867.1	402.1
Reclassification out of Accumulated Other Comprehensive Income | Net Unrealized Gain on Securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Other Net Realized Investment Gain (Loss)									15.3	(12.2)	12.6
Other-Than-Temporary Impairment Loss									(30.5)	(32.4)	(13.5)
Income Before Income Tax									(15.2)	(44.6)	(0.9)
Income Tax Expense (Benefit)									(5.4)	(15.9)	(0.6)
Net Income									(9.8)	(28.7)	(0.3)
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									53.3	83.4	53.0
Income Tax Expense (Benefit)									18.6	27.1	16.6
Net Income									34.7	56.3	36.4
Reclassification out of Accumulated Other Comprehensive Income | Net Gain on Cash Flow Hedges | Interest Rate Swaps
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Other Net Realized Investment Gain (Loss)									3.2	0.5	4.3
Net Investment Income									54.5	50.2	47.8
Interest and Debt Expense									(1.9)	(1.8)	(1.8)
Reclassification out of Accumulated Other Comprehensive Income | Net Gain on Cash Flow Hedges | Foreign Exchange Contracts
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Other Net Realized Investment Gain (Loss)									(1.4)	36.1	6.9
Net Investment Income									(1.1)	(1.6)	(4.2)
Reclassification out of Accumulated Other Comprehensive Income | Net Actuarial Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Other Expenses									(16.3)	(11.9)	(5.6)
Reclassification out of Accumulated Other Comprehensive Income | Prior Service Credit
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Other Expenses									0.5	0.3	1.7
Reclassification out of Accumulated Other Comprehensive Income | Pension Settlement Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Other Expenses									0.0	0.0	(64.4)
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									(15.8)	(11.6)	(68.3)
Income Tax Expense (Benefit)									(5.5)	(4.0)	(23.8)
Net Income									$ (10.3)	$ (7.6)	$ (44.5)