Claim, Claim Adjustment Expense and Future Policy Benefit Reserves - Schedule of Net Strengthening or Releasing of Prior Accident Year Reserves (Medical Professional Liability) (Detail) - Property and Casualty Operations [Member] - Medical Professional 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	$ 75	$ 47	$ 30
2010 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	13	(4)	(5)	$ (30)	$ (20)	$ (11)	$ 3	$ 11	$ 10
Net Strengthening or (Releases) of Prior Accident Year Reserves	(33)
2011 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	2		3	(5)	(29)	25	6	$ 8
Net Strengthening or (Releases) of Prior Accident Year Reserves	10
2012 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	6	9	(9)	(5)	(10)	39	$ 5
Net Strengthening or (Releases) of Prior Accident Year Reserves	35
2013 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	10	10	12	13	21	$ 17
Net Strengthening or (Releases) of Prior Accident Year Reserves	83
2014 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	(6)	5	(7)	48	$ 39
Net Strengthening or (Releases) of Prior Accident Year Reserves	79
2015 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	(6)	(16)	11	$ 66
Net Strengthening or (Releases) of Prior Accident Year Reserves	55
2016 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	14	(2)	60
Net Strengthening or (Releases) of Prior Accident Year Reserves	72
2017 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	9	37
Net Strengthening or (Releases) of Prior Accident Year Reserves	46
Short-duration Insurance Contracts, Accident Year 2018 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	25
Net Strengthening or (Releases) of Prior Accident Year Reserves	25
Total net development for the accident years presented above [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	67	39	65
Total net development for the accident years prior to 2010 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	6	9	(28)
Unallocated Claim Adjustment Expense Development [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	$ 2	$ (1)	$ (7)