Claim and Claim Adjustment Expense Reserves - Schedule of Gross and Net Carried Claim and Claim Adjustment Expense Reserves (Detail) - CNA Financial [Member] - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	$ 11,860	$ 12,102
Gross IBNR Reserves	10,483	10,561
Total Gross Carried Claim and Claim Adjustment Expense Reserves	22,343	22,663	$ 23,271	$ 24,089
Net Case Reserves	9,627	9,836
Net IBNR Reserves	8,622	8,740
Total Net Carried Claim and Claim Adjustment Expense Reserves	18,249	18,576	$ 18,927	$ 19,117
Property and Casualty Operations [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	7,164	7,608
Gross IBNR Reserves	9,207	9,191
Total Gross Carried Claim and Claim Adjustment Expense Reserves	16,371	16,799
Net Case Reserves	6,582	6,992
Net IBNR Reserves	8,328	8,371
Total Net Carried Claim and Claim Adjustment Expense Reserves	14,910	15,363
Non Core Operations [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	4,696	4,494
Gross IBNR Reserves	1,276	1,370
Total Gross Carried Claim and Claim Adjustment Expense Reserves	5,972	5,864
Net Case Reserves	3,045	2,844
Net IBNR Reserves	294	369
Total Net Carried Claim and Claim Adjustment Expense Reserves	$ 3,339	$ 3,213