Summary of Significant Accounting Policies (details) (USD $)	12 Months Ended
	Dec. 31, 2012 item	Dec. 31, 2011	Dec. 31, 2010
Summary of Significant Accounting Policies disclosure
Maximum useful life for buildings held in real estate investments (in years)	39 years
Original maturity of short-term securities	Short-term securities have an original maturity of less than one year
Availability of financial information provided by private equity and real estate partnerships	The private equity and real estate partnerships provide financial information quarterly which is generally available to investors, including the Company, within three to six months following the date of the reporting period.
Availability of financial information provided by hedge funds	The hedge funds provide financial information monthly, which is generally available to investors within one month following the date of the reporting period.
Estimated recovery time for securities for which the issuer is in bankruptcy (in months)	0 years 12 months
Estimated recovery time for securities for which the issuer is financially troubled but not in bankruptcy (in months)	0 years 24 months
Minimum collateral provided by borrowers of securities, as a percentage of the market value of the loaned securities plus accrued interest	102.00%
Liabilities for losses for most long-term disability and annuity claim payments primarily arising from workers' compensation insurance and workers' compensation excess insurance policies	$ 2,010,000,000	$ 2,200,000,000
Liabilities for losses for most long-term disability and annuity claim payments primarily arising from workers' compensation insurance and workers' compensation excess insurance policies, discount rate (percent)	5.00%	5.00%
Liability for guaranty fund and other insurance-related assessments	297,000,000	293,000,000
Recoverables for liability for guaranty fund and other insurance-related assessments	15,000,000	19,000,000
Expected payment period for loss-based assessments	The loss-based assessments are expected to be paid over a period ranging from one year to the life expectancy of certain workers' compensation claimants and the recoveries are expected to occur over the same period of time.
Net written premiums for participating dividend policies as a percent of total Company net written premiums	2.00%	1.00%	1.00%
Liability accrued for policyholder dividends	$ 59,000,000	$ 54,000,000
Number of reportable business segments	3
Number of groups comprising Business Insurance Core	6
Description of relationship with Lloyds's syndicate	International, through its Lloyd's syndicate (Syndicate 5000), for which the Company provides 100% of the capital, underwrites through five principal business units -- marine, global property, accident & special risks, power & utilities and aviation.
Percent of common stock owned, J. Malucelli - Brazilian joint venture	49.50%
Prime [Member] | Minimum [Member]
Key assumptions used in estimation of present value of structured fixed maturity securities
Voluntary prepayment rates	2.00%
Percentage of remaining pool liquidated due to defaults	2.00%
Loss severity	25.00%
Prime [Member] | Maximum [Member]
Key assumptions used in estimation of present value of structured fixed maturity securities
Voluntary prepayment rates	35.00%
Percentage of remaining pool liquidated due to defaults	63.00%
Loss severity	65.00%
Alt-A [Member] | Minimum [Member]
Key assumptions used in estimation of present value of structured fixed maturity securities
Voluntary prepayment rates	0.00%
Percentage of remaining pool liquidated due to defaults	13.00%
Loss severity	40.00%
Alt-A [Member] | Maximum [Member]
Key assumptions used in estimation of present value of structured fixed maturity securities
Voluntary prepayment rates	16.00%
Percentage of remaining pool liquidated due to defaults	70.00%
Loss severity	73.00%
Sub-Prime [Member] | Minimum [Member]
Key assumptions used in estimation of present value of structured fixed maturity securities
Voluntary prepayment rates	1.00%
Percentage of remaining pool liquidated due to defaults	24.00%
Loss severity	65.00%
Sub-Prime [Member] | Maximum [Member]
Key assumptions used in estimation of present value of structured fixed maturity securities
Voluntary prepayment rates	10.00%
Percentage of remaining pool liquidated due to defaults	80.00%
Loss severity	92.00%