Income Taxes (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Net deferred income taxes by jurisdiction
Net deferred income tax assets	$ 15.5	$ 13.0
Deferred income taxes	(533.4)	(409.9)
Total net deferred income tax assets (liabilities)	(517.9)	(396.9)
Current income taxes (benefits):
U.S. federal	115.4	74.1	97.7
State and foreign	51.4	34.2	19.4
Total current income taxes (benefits)	166.8	108.3	117.1
Deferred income taxes (benefits)	69.6	15.8	(17.0)
Income taxes (benefits)	236.4	124.1	100.1
Reconciliation between the U.S. corporate income tax rate and the effective income tax rate from continuing operations
U.S. corporate statutory income tax rate (as a percent)	35.00%	35.00%	35.00%
Dividends received deduction (as a percent)	(8.00%)	(10.00%)	(10.00%)
Impact of equity method presentation (as a percent)	(4.00%)	(6.00%)	(5.00%)
Interest exclusion from taxable income (as a percent)	(3.00%)	(3.00%)	(3.00%)
Impact of court ruling on some uncertain tax positions (as a percent)	6.00%
Other income tax rate impacts (as a percent)	(2.00%)	(1.00%)	(4.00%)
Effective income tax rate (as a percent)	24.00%	15.00%	13.00%
Changes in Unrecognized Tax Benefits
Beginning balance, Unrecognized tax benefits	54.8	54.5
Additions based on tax positions related to the current year	1.5	1.5
Additions for tax positions of prior years	67.1	1.2
Reductions for tax positions related to the current year	(1.8)	(2.4)
Reductions for tax positions of prior years	(7.3)
Ending balance, Unrecognized tax benefits	114.3	54.8	54.5
Amount of unrecognized tax benefits that would reduce the effective income tax rate, if recognized	75.8
Accumulated pre-tax interest and penalties related to unrecognized tax benefits	43.8	23.4
Deferred income tax assets:
Insurance liabilities	80.7	245.2
Investments, including derivative liabilities	659.2	622.0
Net operating and capital loss carryforwards	358.6	326.4
Postretirement benefits	503.3	327.8
Stock-based compensation	72.6	71.0
Other deferred income tax assets	64.4	15.2
Gross deferred income tax assets	1,738.8	1,607.6
Valuation allowance	(1.3)	(1.0)
Total deferred income tax assets	1,737.5	1,606.6
Deferred income tax liabilities:
Deferred income tax liabilities: Deferred policy acquisition costs	(1,051.7)	(1,084.6)
Investments, including derivative assets	(488.9)	(425.7)
Net unrealized gains on available-for-sale securities	(423.3)	(188.6)
Deferred income tax liabilities: Real estate	(103.3)	(115.6)
Intangible assets	(144.6)	(108.1)
Other deferred income tax liabilities	(43.6)	(80.9)
Total deferred income tax liabilities	(2,255.4)	(2,003.5)
Total net deferred income tax assets (liabilities)	(517.9)	(396.9)
Tax return examinations and court rulings
Payment of IRS deficiency, including interest, for 2004 and 2005	62.1
Payment of IRS deficiency, including interest, for 2006 and 2008	46.7
Impact of court ruling on uncertain tax positions	(68.9)
Low end of range of reasonably possible increase (decrease) in unrecognized tax benefits within the next twelve months	0
Upper end of range of reasonably possible increase (decrease) in unrecognized tax benefits within the next twelve months	(28.5)
Deferred income tax disclosures
Accumulated net operating losses attributed to captive reinsurance companies	931.5	640.8
Accumulated but undistributed earnings from operations of foreign subsidiaries for which U.S. federal and state deferred income taxes have not been provided	585.5
Excess book carrying value over tax basis with respect to the original investment of foreign subsidiaries for which U.S. federal and state deferred income taxes have not been provided	106.2
Valuation allowance provided for the deferred tax asset attributable to net operating losses of captive reinsurance companies	0
Valuation allowance provided for the deferred income tax asset attributable to unrealized losses on available-for-sale securities	0
Current income tax receivables associated with outstanding audits
Current income tax (receivables) payables associated with outstanding audit issues	(263.2)	(230.9)
U.S.
Net deferred income taxes by jurisdiction
Deferred income taxes	(381.2)	(264.1)
State
Net deferred income taxes by jurisdiction
Net deferred income tax assets	2.9	1.8
Deferred income tax disclosures-operating loss carryforwards
Net operating loss carryforwards	282.7
International
Net deferred income taxes by jurisdiction
Net deferred income tax assets	12.6	11.2
Deferred income taxes	(152.2)	(145.8)
Deferred income tax disclosures-operating loss carryforwards
Net operating loss carryforwards	$ 59.0