Claim and Claim Adjustment Expense Reserves - Additional Information (Detail) - CNA Financial [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2010	Dec. 31, 1996
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Catastrophe losses, net of reinsurance	$ 165	$ 141	$ 156
Net A&EP adverse development before consideration of LPT	200	150
Fair value of the collateral trust account	2,800	2,800
Property and Casualty Operations [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Favorable net prior year development	316	218	50
Asbestos and Environmental Pollution Reserves [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net A&EP claim and allocated claim adjustment expense reserves				$ 1,600
Aggregate limit under A&EP Loss Portfolio Transfer				4,000
Ceded A&EP claim and allocated claim adjustment expense reserves				1,200
Reinsurance premium paid to NICO under A&EP Loss Portfolio Transfer				2,000
Net reinsurance receivables transferred to NICO under A&EP Loss Portfolio Transfer				215
Total consideration				$ 2,200
Cumulative amounts ceded under the Loss Portfolio Transfer	2,800	2,600
Remaining unrecognized retroactive reinsurance benefit	334	241
Non Core Operations [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Favorable net prior year development	$ 43	$ 50	$ 17
Workers Compensation Reinsurance Pool [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Favorable net prior year development					$ 26