Claim and Claim Adjustment Expense Reserves - Summary of Gross and Net Carried Reserves (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	$ 12,262	$ 12,668
Gross IBNR Reserves	11,827	12,095
Total Gross Carried Claim and Claim Adjustment Expense Reserves	24,089	24,763	24,303	25,496
Net Case Reserves	10,176	10,356
Net IBNR Reserves	8,941	9,281
Total Net Carried Claim and Claim Adjustment Expense Reserves	19,117	19,637	19,283	19,374
CNA Specialty [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	2,270	2,292
Gross IBNR Reserves	4,419	4,456
Total Gross Carried Claim and Claim Adjustment Expense Reserves	6,689	6,748
Net Case Reserves	2,024	2,008
Net IBNR Reserves	4,142	4,104
Total Net Carried Claim and Claim Adjustment Expense Reserves	6,166	6,112
CNA Commercial [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	5,829	6,146
Gross IBNR Reserves	4,820	5,180
Total Gross Carried Claim and Claim Adjustment Expense Reserves	10,649	11,326
Net Case Reserves	5,358	5,611
Net IBNR Reserves	4,269	4,600
Total Net Carried Claim and Claim Adjustment Expense Reserves	9,627	10,211
Life & Group Non-Core [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	2,748	2,690
Gross IBNR Reserves	310	316
Total Gross Carried Claim and Claim Adjustment Expense Reserves	3,058	3,006
Net Case Reserves	2,352	2,253
Net IBNR Reserves	271	275
Total Net Carried Claim and Claim Adjustment Expense Reserves	2,623	2,528
Other Insurance [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	1,415	1,540
Gross IBNR Reserves	2,278	2,143
Total Gross Carried Claim and Claim Adjustment Expense Reserves	3,693	3,683
Net Case Reserves	442	484
Net IBNR Reserves	259	302
Total Net Carried Claim and Claim Adjustment Expense Reserves	$ 701	$ 786