Claim and Claim Adjustment Expense Reserves - Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total pretax (favorable) unfavorable development	$ (132)	$ (175)	$ (282)	$ (187)
CNA Financial [Member] | Specialty [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Medical professional liability	13	(19)	(17)	(11)
Other professional liability and management liability	(48)	(37)	(98)	(41)
Surety	(63)	(70)	(63)	(69)
Warranty	2		7	1
Other	(16)	(4)	(40)	(21)
Total pretax (favorable) unfavorable development	(112)	(130)	(211)	(141)
CNA Financial [Member] | Commercial [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Commercial auto	(12)		(47)	7
General liability	14	3	(38)	8
Workers' compensation	(6)	(1)	48	22
Property and other	(1)	(13)		(37)
Total pretax (favorable) unfavorable development	(5)	(11)	(37)
CNA Financial [Member] | International [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Medical professional liability	(2)	(8)	(3)	(8)
Other professional liability	(1)	(11)	16	(16)
General liability	(2)	(5)	(21)	(12)
Property & marine	(9)	(5)	(16)	(19)
Other	(1)	(5)	(10)	9
Total pretax (favorable) unfavorable development	$ (15)	$ (34)	$ (34)	$ (46)