Claim and Claim Adjustment Expense Reserves - Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total pretax (favorable) unfavorable development	$ (210)	$ (21)	$ (118)
CNA Financial [Member] | Specialty [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Medical professional liability	(43)	39	(27)
Other professional liability and management liability		(87)	(73)
Surety	(69)	(82)	(74)
Warranty	(2)	(2)	(3)
Other	(27)	(4)	(19)
Total pretax (favorable) unfavorable development	(141)	(136)	(196)
CNA Financial [Member] | International [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Medical professional liability	(9)	(7)	(7)
Other professional liability	(16)	(26)	(30)
General liability	(17)	(13)	(8)
Property & marine	(29)	(14)	13
Other	17	(9)	(17)
Commutations		10	11
Total pretax (favorable) unfavorable development	(54)	(59)	(38)
CNA Financial [Member] | Commercial [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Commercial auto	(22)	31	18
General liability	(33)	45	64
Workers' compensation	80	139	91
Property and other	(40)	(39)	(51)
Total pretax (favorable) unfavorable development	$ (15)	$ 176	$ 122