Schedule II. Condensed Financial Information of Registrant (Parent Company) (Schedule of Condensed Financial Information of Registrant, Balance Sheets) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Assets
Fixed maturity securities available-for-sale, at fair value (amortized cost of $2 and $2)	$ 42,604	$ 39,931
Short term investments	1,832	1,641
Other assets	773	795
Total assets	58,522	55,110
Liabilities
Short term debt	13	83
Long term debt	2,557	2,525
Other liabilities	3,245	2,975
Total liabilities	46,208	43,622
Equity
Common stock ($2.50 par value; 500,000,000 shares authorized; 273,040,243 shares issued; 269,399,390 and 269,274,900 shares outstanding)	683	683
Additional paid-in capital	2,146	2,141
Retained earnings	8,774	8,308
Accumulated other comprehensive income (loss)	831	480
Treasury stock (3,640,853 and 3,765,343 shares), at cost	(99)	(102)
Notes receivable for the issuance of common stock	(21)	(22)
Total CNA stockholders’ equity	12,314	11,488
Total liabilities and equity	58,522	55,110
Balance Sheet Parenthetical
Common stock, par value	$ 2.50	$ 2.50
Common stock, shares authorized	500,000,000	500,000,000
Common stock, shares issued	273,040,243	273,040,243
Common stock, shares outstanding	269,399,390	269,274,900
Treasury stock, shares	3,640,853	3,765,343
Parent Company [Member]
Assets
Investment in subsidiaries	14,427	13,495
Fixed maturity securities available-for-sale, at fair value (amortized cost of $2 and $2)	2	2
Short term investments	448	292
Amounts due from subsidiaries	2
Surplus note due from subsidiary		250
Other assets	5	18
Total assets	14,884	14,057
Liabilities
Short term debt	3	3
Long term debt	2,527	2,525
Other liabilities	40	41
Total liabilities	2,570	2,569
Equity
Common stock ($2.50 par value; 500,000,000 shares authorized; 273,040,243 shares issued; 269,399,390 and 269,274,900 shares outstanding)	683	683
Additional paid-in capital	2,146	2,141
Retained earnings	8,774	8,308
Accumulated other comprehensive income (loss)	831	480
Treasury stock (3,640,853 and 3,765,343 shares), at cost	(99)	(102)
Notes receivable for the issuance of common stock	(21)	(22)
Total CNA stockholders’ equity	12,314	11,488
Total liabilities and equity	14,884	14,057
Balance Sheet Parenthetical
Marketable securities fixed maturities available-for-sale at amortized cost basis	$ 2	$ 2
Common stock, par value	$ 2.50	$ 2.50
Common stock, shares authorized	500,000,000	500,000,000
Common stock, shares issued	273,040,243	273,040,243
Common stock, shares outstanding	269,399,390	269,274,900
Treasury stock, shares	3,640,853	3,765,343