Claim and Claim Adjustment Expense Reserves - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2010
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Catastrophe losses, net of reinsurance	$ 141	$ 156	$ 169
Additional amounts ceded under LPT			$ 900
Fair value of the collateral trust account	2,800	3,400
International [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Commutation of third party capital provider, percentage			15.00%
Percentage of premium, losses and expenses			15.00%
Commercial [Member] | CNA Financial [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Favorable development in accident years	80	139	$ 91
1996 and Prior [Member] | Commercial [Member] | CNA Financial [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Favorable development in accident years		26
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	$ 2,200
Cumulative amounts ceded under the Loss Portfolio Transfer	2,600	2,500
Remaining unrecognized retroactive reinsurance benefit	241	176
Life & Group Non-Core [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Favorable net prior year development	$ 50	$ 14	$ 9