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, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	$ 11,395	$ 11,670
Gross IBNR Reserves	10,589	10,334
Total Gross Carried Claim and Claim Adjustment Expense Reserves	21,984	22,004	$ 22,343	$ 22,663
Net Case Reserves	9,523	9,645
Net IBNR Reserves	8,442	8,425
Total Net Carried Claim and Claim Adjustment Expense Reserves	17,965	18,070	$ 18,249	$ 18,576
Property and Casualty Operations [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	6,671	6,913
Gross IBNR Reserves	9,287	9,156
Total Gross Carried Claim and Claim Adjustment Expense Reserves	15,958	16,069
Net Case Reserves	6,063	6,343
Net IBNR Reserves	8,290	8,232
Total Net Carried Claim and Claim Adjustment Expense Reserves	14,353	14,575
Other Insurance Operations [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	4,724	4,757
Gross IBNR Reserves	1,302	1,178
Total Gross Carried Claim and Claim Adjustment Expense Reserves	6,026	5,935
Net Case Reserves	3,460	3,302
Net IBNR Reserves	152	193
Total Net Carried Claim and Claim Adjustment Expense Reserves	$ 3,612	$ 3,495