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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)

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