Summary of Significant Accounting Policies (Narrative) (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Jun. 30, 2011
Payments to acquire additional interest in subsidiaries	$ 475,000,000
Investments in life settlement contracts	117,000,000
Basis of Presentation [Member]
Noncontrolling interest, ownership percentage by parent	90.00%
CNA Surety [Member]
Previous ownership of outstanding publicly-traded stock of CNA Surety				61.00%
Payments to acquire additional interest in subsidiaries	475,000,000
Offer price per share for CNA Surety	$ 26.55
Noncontrolling interest, decrease from redemptions or purchase of interests	434,000,000
Net income attributable to the noncontrolling interest in CNA Surety	12,000,000	52,000,000	46,000,000
First Insurance Company of Hawaii (FICOH) [Member]
Sale of CNA ownership percentage of FICOH	50.00%
Claim and claim adjustment expense reserves [Member]
Anticipated amounts due from insureds related to losses under deductible policies	1,400,000,000	1,400,000,000
Minimum interest rate used in determining present value of obligations of structured settlements not funded by annuities	5.50%	4.60%
Maximum interest rate used in determining present value of obligations of structured settlements not funded by annuities	8.00%	7.50%
Discounted reserves for unfunded structured settlements	632,000,000	713,000,000
Discounted reserves for unfunded structure settlements, discount amount	1,100,000,000	1,100,000,000
Minimum interest rate used to discount workers' compensation lifetime claim reserves and accident and health claim reserves	3.00%	3.00%
Maximum interest rate used to discount workers' compensation lifetime claim reserves and accident and health claim reserves	6.50%	6.50%
Discounted reserves for worker's compensation lifetime claim reserves and accident and health claim reserves	2,100,000,000	1,900,000,000
Discounted reserves for worker's compensation lifetime claim reserves and accident and health claim reserves, discount amount	520,000,000	487,000,000
Future policy benefits reserves [Member]
Minimum interest rate used to calculate reserves for long term care products	5.00%	6.00%
Maximum interest rate used to calculate reserves for long term care products	7.50%	7.60%
Minimum interest rate used to calculate reserves for payout annuity contracts	5.40%	2.80%
Maximum interest rate used to calculate reserves for payout annuity contracts	7.50%	10.20%
Increase in insurance reserves due to unlocking actuarial assumptions related to payout annuity contracts	166,000,000
Guaranty fund and other insurance-related assessments [Member]
Liability balance for guaranty fund	152,000,000	160,000,000
Related assets for premium tax offsets included in other assets	2,000,000	3,000,000
Reinsurance [Member]
Percentage of billed receivables compared to total reinsurance receivables	less than 5%
Deposit assets	18,000,000	23,000,000
Deposit liabilities	123,000,000	114,000,000
Deferred acquistion costs [Member]
Deferred acquistion costs, shadow adjustment	412,000,000
Investments in life settlement contracts and related revenue recognition [Member]
Investments in life settlement contracts	117,000,000	129,000,000
Increase in fair value recognized on life settlement contracts	5,000,000	10,000,000	10,000,000
Gain recognized on matured life settlement contracts	28,000,000	19,000,000	24,000,000
Seperate account business [Member]
Increase (decrease) in policyholders' funds, pension deposit guarantee liability	18,000,000	(24,000,000)	(42,000,000)
Valuation of investments [Member]
Shadow adjustment, net of participating policyholders' interest and tax	582,000,000	150,000,000
Reduction of net unrealized gains on investments included in AOCI due to shadow adjustments	732,000,000	150,000,000
Pension and Postretirement Benefits [Member]
New accounting estimate, effect of change on net income			20,000,000
Stock-Based Compensation [Member]
Requisite service period for stock-based compensation expense	four
Foreign Currency [Member]
Foreign currency transaction gain (loss), before tax	4,000,000	(19,000,000)	(14,000,000)
Property and Equipment [Member]
Useful life of furniture and fixtures	seven
Useful life for office equipment	five
Useful life for data processing equipment and software minimum	three
Useful life for data processing equipment and software maximum	five
Useful life of company owned buildings	fifty
Earnings (Loss) Per Share Data [Member]
Weighted average number diluted shares outstanding adjustment	290,000	380,000	120,000
Antidilutive securities excluded from computation of earnings per share, amount	1,100,000	1,200,000	1,700,000
Supplementary Cash Flow Information [Member]
Interest paid, net	175,000,000	145,000,000	124,000,000
Income taxes paid	61,000,000
Proceeds from income tax refunds		175,000,000	117,000,000
Accounting standards update [Member]
Reduction of stockholders' equity due to adoption of updated accounting guidance for costs associated with acquiring or renewing insurance contracts	$ 70,000,000