Claim, Claim Adjustment Expense and Future Policy Benefit Reserves - Schedule of Gross and Net Carried Claim and Claim Adjustment Expense Reserves (Detail) - CNA Financial Corporation [Member] - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	$ 10,989	$ 11,395
Gross IBNR Reserves	10,731	10,589
Total Gross Carried Claim and Claim Adjustment Expense Reserves	21,720	21,984	$ 22,004	$ 22,343
Net Case Reserves	9,178	9,523
Net IBNR Reserves	8,707	8,442
Total Net Carried Claim and Claim Adjustment Expense Reserves	17,885	17,965	$ 18,070	$ 18,249
Property and Casualty Operations [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	6,276	6,671
Gross IBNR Reserves	9,494	9,287
Total Gross Carried Claim and Claim Adjustment Expense Reserves	15,770	15,958
Net Case Reserves	5,645	6,063
Net IBNR Reserves	8,508	8,290
Total Net Carried Claim and Claim Adjustment Expense Reserves	14,153	14,353
Other Insurance Operations [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	4,713	4,724
Gross IBNR Reserves	1,237	1,302
Total Gross Carried Claim and Claim Adjustment Expense Reserves	5,950	6,026
Net Case Reserves	3,533	3,460
Net IBNR Reserves	199	152
Total Net Carried Claim and Claim Adjustment Expense Reserves	$ 3,732	$ 3,612