Claim and Claim Adjustment Expense Reserves - Schedule of Net Strengthening or Releasing of Prior Accident Year Reserves (Workers' Compensation) (Detail) - Property and Casualty Operations [Member] - Workers' Compensation [Member] - CNA Financial [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	$ 150	$ 80	$ 139
2007 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	19	(2)	6	$ 1		$ (1)	$ 8	$ 16	$ 12
Net Strengthening or (Releases) of Prior Accident Year Reserves	59
2008 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	8	8	10	4	$ 2	13	18	$ 17
Net Strengthening or (Releases) of Prior Accident Year Reserves	80
2009 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	8	6	11	4	2	7	$ 4
Net Strengthening or (Releases) of Prior Accident Year Reserves	42
2010 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	20	14	20	22	22	$ 43
Net Strengthening or (Releases) of Prior Accident Year Reserves	141
2011 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	24	(6)	11	9	$ 35
Net Strengthening or (Releases) of Prior Accident Year Reserves	73
2012 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	10	13	32	$ 27
Net Strengthening or (Releases) of Prior Accident Year Reserves	82
2013 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	26	21	35
Net Strengthening or (Releases) of Prior Accident Year Reserves	82
2014 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves		15
Net Strengthening or (Releases) of Prior Accident Year Reserves	15
2015 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	10
Net Strengthening or (Releases) of Prior Accident Year Reserves	10
Total net development for the accident years presented above [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	125	69	125
Adjustment for Development on a Discounted Basis [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	1	(4)	1
Total Net Development for Accident Years Prior to 2007 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	$ 24	$ 15	$ 13