Claim and Claim Adjustment Expense Reserves - Reconciliation of Net Liability for Unpaid Claim and Claim Adjustment Expenses for Property and Casualty Operations to Amount Presented in Consolidated Balance Sheets (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total reinsurance receivables	$ 4,094	$ 4,087
Total gross liability for unpaid claims and claims adjustment expenses	22,343	22,663
Property and Casualty Operations [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total reinsurance receivables	1,461
Non Core Operations [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total reinsurance receivables	2,633
CNA Financial [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total net claim and claim adjustment expenses	18,249	18,576	$ 18,927	$ 19,117
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	14,910	15,363
CNA Financial [Member] | Non Core Operations [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total net claim and claim adjustment expenses	$ 3,339	$ 3,213