Claim and Claim Adjustment Expense Reserves - Net Prior Year 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]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development	$ (210)	$ (21)	$ (118)
Pretax (favorable) unfavorable premium development	(8)	(32)	(42)
Total pretax (favorable) unfavorable net prior year development	(218)	(53)	(160)
CNA Financial [Member] | Specialty [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development	(141)	(136)	(196)
Pretax (favorable) unfavorable premium development	(11)	(13)	(14)
Total pretax (favorable) unfavorable net prior year development	(152)	(149)	(210)
CNA Financial [Member] | Commercial [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development	(15)	176	122
Pretax (favorable) unfavorable premium development	(15)	(20)	(8)
Total pretax (favorable) unfavorable net prior year development	(30)	156	114
CNA Financial [Member] | International [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development	(54)	(59)	(38)
Pretax (favorable) unfavorable premium development	18	2	(21)
Total pretax (favorable) unfavorable net prior year development	$ (36)	(57)	(59)
Other [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development		(2)	(6)
Pretax (favorable) unfavorable premium development		(1)	1
Total pretax (favorable) unfavorable net prior year development		$ (3)	$ (5)