Claim and Claim Adjustment Expense 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 [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	$ (37)	$ (43)	$ 39
2007 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	(14)	(2)	11	$ (1)	$ (4)	$ (32)	$ (17)	$ (8)	$ 4
Net Strengthening or (Releases) of Prior Accident Year Reserves	(63)
2008 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	(13)	(12)	(1)		(12)	(16)	45	$ 25
Net Strengthening or (Releases) of Prior Accident Year Reserves	16
2009 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	(31)	(8)	(9)	(26)	12	25	$ 7
Net Strengthening or (Releases) of Prior Accident Year Reserves	(30)
2010 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	(43)	(24)	(16)	8		$ (5)
Net Strengthening or (Releases) of Prior Accident Year Reserves	(80)
2011 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	(10)	(32)	26	15	$ 6
Net Strengthening or (Releases) of Prior Accident Year Reserves	5
2012 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	(8)	(8)	46	$ 3
Net Strengthening or (Releases) of Prior Accident Year Reserves	33
2013 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	7	20	6
Net Strengthening or (Releases) of Prior Accident Year Reserves	33
2014 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	37	37
Net Strengthening or (Releases) of Prior Accident Year Reserves	74
2015 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	59
Net Strengthening or (Releases) of Prior Accident Year Reserves	59
Total net development for the accident years presented above [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	(16)	(29)	63
Total Net Development for Accident Years Prior to 2007 [Member]
Claims Development [Line Items]
Strengthening or (Releases) of Prior Accident Year Reserves	$ (21)	$ (14)	$ (24)