Claim and Claim Adjustment Expense Reserves - Net Prior Year Development in CNA's Property and Casualty Operations (Detail) - CNA Financial [Member] - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Medical professional liability	$ 15	$ 3	$ 30	$ 23
Other professional liability and management liability	(7)	(34)	(19)	(68)
Surety	(15)	(15)	(40)	(30)
Commercial auto	(3)		(8)	(1)
General liability	13	26	(7)	18
Workers' compensation	(7)	(6)	(5)	(12)
Other	(27)	(33)	4	(28)
Total pretax (favorable) unfavorable development	$ (31)	$ (59)	$ (45)	$ (98)