Claim and Claim Adjustment Expense Reserves - Schedule of Net Incurred Claims and Allocated Claim Adjustment Expenses (Surety) (Detail) - Property and Casualty Operations [Member] - Surety [Member] - CNA Financial [Member] $ in Millions	Dec. 31, 2017 USD ($) Claim	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)	Dec. 31, 2009 USD ($)	Dec. 31, 2008 USD ($)
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 775
IBNR	305
2008 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 44	$ 44	$ 45	$ 48	$ 52	$ 61	$ 68	$ 73	$ 114	$ 114
Cumulative number of claims | Claim	7,199
2009 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 53	53	52	59	68	85	103	114	$ 114
IBNR	$ 1
Cumulative number of claims | Claim	6,679
2010 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 59	63	66	76	84	111	112	$ 112
IBNR	$ 7
Cumulative number of claims | Claim	5,962
2011 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 66	70	75	87	116	121	$ 120
IBNR	$ 9
Cumulative number of claims | Claim	5,795
2012 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 45	52	70	98	122	$ 120
IBNR	$ 9
Cumulative number of claims | Claim	5,519
2013 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 91	106	115	121	$ 120
IBNR	$ 10
Cumulative number of claims | Claim	4,993
2014 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 69	94	124	$ 123
IBNR	$ 25
Cumulative number of claims | Claim	4,938
2015 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 104	131	$ 131
IBNR	$ 63
Cumulative number of claims | Claim	4,670
2016 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 124	$ 124
IBNR	$ 84
Cumulative number of claims | Claim	4,707
2017 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 120
IBNR	$ 97
Cumulative number of claims | Claim	2,901