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Claim and Claim Adjustment Expense Reserves - Net Prior Year Development (Detail) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development $ (132) $ (175) $ (282) $ (187)
Pretax (favorable) unfavorable premium development (5) (5) (27) (11)
Total pretax (favorable) unfavorable net prior year development (137) (180) (309) (198)
CNA Financial [Member] | Specialty [Member]        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development (112) (130) (211) (141)
Pretax (favorable) unfavorable premium development   (2) (18) (10)
Total pretax (favorable) unfavorable net prior year development (112) (132) (229) (151)
CNA Financial [Member] | Commercial [Member]        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development (5) (11) (37)  
Pretax (favorable) unfavorable premium development (3) (5) (7) (17)
Total pretax (favorable) unfavorable net prior year development (8) (16) (44) (17)
CNA Financial [Member] | International [Member]        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development (15) (34) (34) (46)
Pretax (favorable) unfavorable premium development (2) 2 (2) 16
Total pretax (favorable) unfavorable net prior year development $ (17) $ (32) $ (36) $ (30)