Claim And Claim Adjustment Expense Reserves (Summary Of Gross And Net Carried Reserves) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Claim And Claim Adjustment Expense Reserves [Line Items]
Gross Case Reserves	$ 12,538	$ 12,564
Gross IBNR Reserves	11,765	12,932
Net Case Reserves	10,178	9,633
Net IBNR Reserves	9,105	9,741
Total Gross Carried Claim and Claim Adjustment Expense Reserves	24,303	25,496	26,816	27,593
Total Net Carried Claim and Claim Adjustment Expense Reserves	19,283	19,374	21,222	21,305
CNA Specialty [Member]
Claim And Claim Adjustment Expense Reserves [Line Items]
Gross Case Reserves	2,441	2,341
Gross IBNR Reserves	4,399	4,452
Net Case Reserves	2,086	1,992
Net IBNR Reserves	3,937	3,926
Total Gross Carried Claim and Claim Adjustment Expense Reserves	6,840	6,793
Total Net Carried Claim and Claim Adjustment Expense Reserves	6,023	5,918
CNA Commercial [Member]
Claim And Claim Adjustment Expense Reserves [Line Items]
Gross Case Reserves	6,266	6,390
Gross IBNR Reserves	5,243	6,132
Net Case Reserves	5,720	5,349
Net IBNR Reserves	4,670	5,292
Total Gross Carried Claim and Claim Adjustment Expense Reserves	11,509	12,522
Total Net Carried Claim and Claim Adjustment Expense Reserves	10,390	10,641
Life And Group Non-Core [Member]
Claim And Claim Adjustment Expense Reserves [Line Items]
Gross Case Reserves	2,510	2,403
Gross IBNR Reserves	315	336
Net Case Reserves	2,025	1,831
Net IBNR Reserves	254	266
Total Gross Carried Claim and Claim Adjustment Expense Reserves	2,825	2,739
Total Net Carried Claim and Claim Adjustment Expense Reserves	2,279	2,097
Other Insurance [Member]
Claim And Claim Adjustment Expense Reserves [Line Items]
Gross Case Reserves	1,321	1,430
Gross IBNR Reserves	1,808	2,012
Net Case Reserves	347	461
Net IBNR Reserves	244	257
Total Gross Carried Claim and Claim Adjustment Expense Reserves	3,129	3,442
Total Net Carried Claim and Claim Adjustment Expense Reserves	$ 591	$ 718