Claim and Claim Adjustment Expense Reserves - Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total pretax (favorable) unfavorable development	$ (98)	$ (5)	$ (150)	$ (12)
CNA Financial [Member] | Specialty [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Medical professional liability	(23)	(6)	(30)	8
Other professional liability and management liability	(41)	(1)	(50)	(4)
Surety				1
Warranty	3	1	5	1
Other	(4)	(7)	(24)	(17)
Total pretax (favorable) unfavorable development	(65)	(13)	(99)	(11)
CNA Financial [Member] | Commercial [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Commercial auto	(20)	7	(35)	7
General liability	(37)	1	(52)	5
Workers' compensation	50	24	54	23
Property and other	(11)	(16)	1	(24)
Total pretax (favorable) unfavorable development	(18)	16	(32)	11
CNA Financial [Member] | International [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Medical professional liability	(1)		(1)
Other professional liability	18	(5)	17	(5)
General liability	(19)	(2)	(19)	(7)
Property & marine	(3)	(8)	(7)	(14)
Other	(10)	7	(9)	14
Total pretax (favorable) unfavorable development	$ (15)	$ (8)	$ (19)	$ (12)