Claim, Claim Adjustment Expense and Future Policy Benefit Reserves - Schedule of Net Strengthening or Releasing of Prior Accident Year Reserves (General Liability) (Detail) - Property and Casualty Operations [Member] - General Liability [Member] - CNA Financial Corporation [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	Dec. 31, 2011
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	$ 54	$ 32	$ (24)
2010 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	(1)		(6)	$ (29)	$ (24)	$ 41	$ 51	$ (6)	$ 18
Net Strengthening or (Releases) of Prior Accident Year Reserves	44
2011 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	(2)	(1)	(11)	5	(1)	46	42	$ (2)
Net Strengthening or (Releases) of Prior Accident Year Reserves	76
2012 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	(5)		16	(17)	(3)	28	$ 24
Net Strengthening or (Releases) of Prior Accident Year Reserves	43
2013 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	(3)	10	(42)	5	(5)	$ 5
Net Strengthening or (Releases) of Prior Accident Year Reserves	(30)
2014 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	23	4	(23)	(4)	$ 5
Net Strengthening or (Releases) of Prior Accident Year Reserves	5
2015 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	11	15	(2)	$ (5)
Net Strengthening or (Releases) of Prior Accident Year Reserves	19
2016 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	4	8	36
Net Strengthening or (Releases) of Prior Accident Year Reserves	48
2018 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	(9)
Net Strengthening or (Releases) of Prior Accident Year Reserves	(9)
Total net development for the accident years presented above [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	18	36	(32)
Total net development for the accident years prior to 2010 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	29
Unallocated Claim Adjustment Expense Development [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	$ 7	$ (4)	$ 8