Claim and Claim Adjustment Expense Reserves - Reconciliation of Net Liability for Unpaid Claim and Claim Adjustment Expenses to Amount Presented in Consolidated Balance Sheets (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total reinsurance receivables	$ 4,019	$ 3,934
Total gross liability for unpaid claims and claims adjustment expenses	21,984	22,004
Property and Casualty Operations [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total reinsurance receivables	1,605
Other Insurance Operations [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total reinsurance receivables	2,414
CNA Financial [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total net claim and claim adjustment expenses	17,965	18,070	$ 18,249	$ 18,576
CNA Financial [Member] | Property and Casualty Operations [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total net claim and claim adjustment expenses	14,353	$ 14,575
CNA Financial [Member] | Other Insurance Operations [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total net claim and claim adjustment expenses	$ 3,612