Claim, Claim Adjustment Expense and Future Policy Benefit Reserves - Schedule of Net Strengthening or Releasing of Prior Accident Year Reserves (Other Professional Liability and Management Liability) (Detail) - Property and Casualty Operations [Member] - CNA Financial Corporation [Member] - Other Professional Liability and Management Liability [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	$ (69)	$ (127)	$ (126)
2010 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves		(6)	3	$ (13)	$ (10)	$ (1)		$ 20
Net Strengthening or (Releases) of Prior Accident Year Reserves	(7)
2011 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	(3)	(11)	(12)	(33)	(5)	15	$ 26	$ 28
Net Strengthening or (Releases) of Prior Accident Year Reserves	5
2012 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	(2)	(13)	6	(38)	(9)	(22)	$ (14)
Net Strengthening or (Releases) of Prior Accident Year Reserves	(92)
2013 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	(13)	(3)	(19)	(41)	32	$ 10
Net Strengthening or (Releases) of Prior Accident Year Reserves	(34)
2014 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	19	4	(54)	(13)	$ 20
Net Strengthening or (Releases) of Prior Accident Year Reserves	(24)
2015 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	(25)	(45)	(15)	$ 4
Net Strengthening or (Releases) of Prior Accident Year Reserves	(81)
2016 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	4		(1)
Net Strengthening or (Releases) of Prior Accident Year Reserves	3
2017 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	(32)	(2)
Net Strengthening or (Releases) of Prior Accident Year Reserves	(34)
2018 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	14
Net Strengthening or (Releases) of Prior Accident Year Reserves	14
Total net development for the accident years presented above [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	(38)	(76)	(92)
Total net development for the accident years prior to 2010 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	(17)	(44)	(27)
Unallocated Claim Adjustment Expense Development [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	$ (14)	$ (7)	$ (7)