Claim and Claim Adjustment Expense Reserves - Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development (Detail) - CNA Financial [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Medical professional liability	$ (37)	$ (43)	$ 39
Other professional liability and management liability	(130)		(87)
Surety	(63)	(69)	(82)
Commercial auto	(46)	(22)	31
General liability	(28)	(33)	45
Workers' compensation	150	80	139
Other	(134)	(123)	(106)
Total pretax (favorable) unfavorable development	$ (288)	$ (210)	$ (21)