Claim, Claim Adjustment Expense and Future Policy Benefit 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, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total reinsurance receivables	$ 3,835	$ 4,019
Total gross liability for unpaid claims and claims adjustment expenses	21,720	21,984
CNA Financial Corporation [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	17,885	17,965	$ 18,070	$ 18,249
Total reinsurance receivables	3,835
Total gross liability for unpaid claims and claims adjustment expenses	21,720
Property and Casualty Operations [Member] | CNA Financial Corporation [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	14,153	$ 14,353
Total reinsurance receivables	1,617
Other Insurance Operations [Member] | CNA Financial Corporation [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	3,732
Total reinsurance receivables	$ 2,218