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