Claim and Claim Adjustment Expense Reserves - Schedule of Net Strengthening or Releasing of Prior Accident Year Reserves (Surety) (Detail) - Property and Casualty Operations [Member] - Surety [Member] - CNA Financial [Member] - USD ($) $ in Millions	12 Months Ended
	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	$ (70)	$ (84)	$ (63)
2009 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	(1)		1	$ (7)	$ (9)	$ (17)	$ (18)	$ (11)
Net Strengthening or (Releases) of Prior Accident Year Reserves	(62)
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	(6)	(7)	(18)	(28)	(24)	$ 2
Net Strengthening or (Releases) of Prior Accident Year Reserves	(81)
2013 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	(4)	(15)	(9)	(6)	$ 1
Net Strengthening or (Releases) of Prior Accident Year Reserves	(33)
2014 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	(9)	(25)	(30)	$ 1
Net Strengthening or (Releases) of Prior Accident Year Reserves	(63)
2015 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	(25)	(27)
Net Strengthening or (Releases) of Prior Accident Year Reserves	(52)
2016 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	(15)
Net Strengthening or (Releases) of Prior Accident Year Reserves	(15)
2017 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	(5)
Net Strengthening or (Releases) of Prior Accident Year Reserves	(5)
Total net development for the accident years presented above [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	(67)	(82)	(64)
Total net development for the accident years prior to 2009 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	$ (3)	1	$ 1
Unallocated Claim Adjustment Expense Development [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves		$ (3)