Claim, Claim Adjustment Expense and Future Policy Benefit Reserves - Net Prior Year Development in CNA's Property and Casualty Operations (Detail) - CNA Financial Corporation [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Medical professional liability	$ 75	$ 47	$ 30
Other professional liability and management liability	(69)	(127)	(126)
Surety	(92)	(70)	(84)
Commercial auto	(25)	1	(35)
General liability	54	32	(24)
Workers' compensation	(13)	(32)	(63)
Other	(3)	(32)	(6)
Total pretax favorable development	$ (73)	$ (181)	$ (308)