Claim and Claim Adjustment Expense Reserves - Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development (Detail) - CNA Financial [Member] - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Medical professional liability	$ 1	$ (7)
Other professional liability and management liability	(32)	(9)
Commercial auto	(26)	(15)
General liability		(15)
Workers' compensation		4
Other		(10)
Total pretax (favorable) unfavorable development	$ (57)	$ (52)