Claim and Claim Adjustment Expense Reserves - Net Prior Year Development in CNA's Property and Casualty Operations (Detail) - CNA Financial [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Medical professional liability	$ 47	$ 30	$ 9
Other professional liability and management liability	(127)	(126)	(140)
Surety	(70)	(84)	(63)
Commercial auto	1	(35)	(47)
General liability	32	(24)	(65)
Workers' compensation	(32)	(63)	145
Other	(32)	(6)	(127)
Total pretax (favorable) unfavorable development	$ (181)	$ (308)	$ (288)