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Claim and Claim Adjustment Expense Reserves - Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development (Detail) - CNA Financial [Member] - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Medical professional liability $ 1 $ 13 $ 5 $ (17)
Other professional liability and management liability (27) (48) (96) (98)
Surety (82) (63) (82) (63)
Commercial auto (14) (12) (40) (47)
General liability 7 14 6 (38)
Workers' compensation 7 (6) (39) 48
Other (7) (30) 19 (67)
Total pretax (favorable) unfavorable development $ (115) $ (132) $ (227) $ (282)