Claim, Claim Adjustment Expense and Future Policy Benefit Reserves - Schedule of Net Strengthening or Releasing of Prior Accident Year Reserves (Surety) (Detail) - Property and Casualty Operations [Member] - Surety [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
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	$ (92)	$ (70)	$ (84)
2010 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves		2	(4)	$ (3)	$ (10)	$ (8)	$ (27)	$ (1)
Net Strengthening or (Releases) of Prior Accident Year Reserves	(51)
2011 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves		(4)	(4)	(5)	(12)	(29)	(5)	$ 1
Net Strengthening or (Releases) of Prior Accident Year Reserves	(58)
2012 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	(1)	(6)	(7)	(18)	(28)	(24)	$ 2
Net Strengthening or (Releases) of Prior Accident Year Reserves	(82)
2013 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	(4)	(4)	(15)	(9)	(6)	$ 1
Net Strengthening or (Releases) of Prior Accident Year Reserves	(37)
2014 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	(15)	(9)	(25)	$ (30)	$ 1
Net Strengthening or (Releases) of Prior Accident Year Reserves	(78)
2015 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	(16)	(25)	(27)
Net Strengthening or (Releases) of Prior Accident Year Reserves	(68)
2016 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	(25)	(15)
Net Strengthening or (Releases) of Prior Accident Year Reserves	(40)
2017 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	(12)	(5)
Net Strengthening or (Releases) of Prior Accident Year Reserves	(17)
2018 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	(6)
Net Strengthening or (Releases) of Prior Accident Year Reserves	(6)
Total net development for the accident years presented above [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	(79)	(66)	(82)
Total net development for the accident years prior to 2010 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	(3)	$ (4)	1
Unallocated Claim Adjustment Expense Development [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	$ (10)		$ (3)