Claim and Claim Adjustment Expense Reserves - Net Prior Year 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]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development	$ (115)	$ (132)	$ (227)	$ (282)
Pretax (favorable) unfavorable premium development	(19)	(5)	(2)	(27)
Total pretax (favorable) unfavorable net prior year development	$ (134)	$ (137)	$ (229)	$ (309)

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

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