Schedule of Reclassifications from Accumulated Other Comprehensive Income to Income (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Reclassifications from Accumulated Other Comprehensive Income to Income [Line Items]
Income Tax Expense (Benefit)	$ 92.9	$ 80.0	$ 184.1	$ 176.1
Net Gain on Cash Flow Hedges
Reclassifications from Accumulated Other Comprehensive Income to Income [Line Items]
Other Comprehensive Income (Loss), Reclassification Adjustment on Derivatives Included in Net Income, before Tax	4.3		5.5
Cash Flow Hedge Gain (Loss) Reclassified to Earnings	2.8		3.6
Reclassification Out of Accumulated Other Comprehensive Income | Net Unrealized Gain on Securities
Reclassifications from Accumulated Other Comprehensive Income to Income [Line Items]
Other Comprehensive Income (Loss), Reclassification Adjustment for Sale of Securities Included in Net Income, before Tax	5.7		17.6
Other Comprehensive Income (Loss), Reclassification Adjustment for Sale of Securities Included in Net Income, Net of Tax	3.7		11.5
Reclassification Out of Accumulated Other Comprehensive Income | Unrecognized Pension and Postretirement Benefit Costs
Reclassifications from Accumulated Other Comprehensive Income to Income [Line Items]
Other Comprehensive Income (Loss), Reclassification, Pension and Other Postretirement Benefit Plans, Net Gain (Loss) Recognized in Net Periodic Benefit Cost, before Tax	(12.3)		(26.0)
Other Comprehensive Income (Loss), Reclassification, Pension and Other Postretirement Benefit Plans, Net Gain (Loss) Recognized in Net Periodic Benefit Cost, Net of Tax	(7.9)		(16.9)
Net Realized Investments Gain Loss | Reclassification Out of Accumulated Other Comprehensive Income | Net Unrealized Gain on Securities
Reclassifications from Accumulated Other Comprehensive Income to Income [Line Items]
Gain on Sales of Securities and Other Invested Assets	5.7		18.4
Other than Temporary Impairment Losses	0		(0.8)
Net Realized Investments Gain Loss | Interest Rate Swaps | Net Gain on Cash Flow Hedges
Reclassifications from Accumulated Other Comprehensive Income to Income [Line Items]
Cash Flow Hedge Gain (Loss) Reclassified to Earnings	0.8		0.8
Net Realized Investments Gain Loss | Foreign Exchange Contracts | Net Gain on Cash Flow Hedges
Reclassifications from Accumulated Other Comprehensive Income to Income [Line Items]
Cash Flow Hedge Gain (Loss) Reclassified to Earnings	(5.7)		(12.4)
Net Investment Income | Interest Rate Swaps | Net Gain on Cash Flow Hedges
Reclassifications from Accumulated Other Comprehensive Income to Income [Line Items]
Cash Flow Hedge Gain (Loss) Reclassified to Earnings	11.1		21.1
Net Investment Income | Foreign Exchange Contracts | Net Gain on Cash Flow Hedges
Reclassifications from Accumulated Other Comprehensive Income to Income [Line Items]
Cash Flow Hedge Gain (Loss) Reclassified to Earnings	(1.5)		(3.2)
Interest and Debt Expense | Interest Rate Swaps | Net Gain on Cash Flow Hedges
Reclassifications from Accumulated Other Comprehensive Income to Income [Line Items]
Cash Flow Hedge Gain (Loss) Reclassified to Earnings	(0.4)		(0.8)
Other Expense | Reclassification Out of Accumulated Other Comprehensive Income | Unrecognized Pension and Postretirement Benefit Costs
Reclassifications from Accumulated Other Comprehensive Income to Income [Line Items]
Amortization of Net Actuarial Loss	(12.9)		(27.8)
Amortization of Prior Service Credit	1.3		2.5
Curtailment Loss	(0.7)		(0.7)
Income Tax Expense | Net Gain on Cash Flow Hedges
Reclassifications from Accumulated Other Comprehensive Income to Income [Line Items]
Income Tax Expense (Benefit)	1.5		1.9
Income Tax Expense | Reclassification Out of Accumulated Other Comprehensive Income | Net Unrealized Gain on Securities
Reclassifications from Accumulated Other Comprehensive Income to Income [Line Items]
Income Tax Expense (Benefit)	2.0		6.1
Income Tax Expense | Reclassification Out of Accumulated Other Comprehensive Income | Unrecognized Pension and Postretirement Benefit Costs
Reclassifications from Accumulated Other Comprehensive Income to Income [Line Items]
Income Tax Expense (Benefit)	$ (4.4)		$ (9.1)