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, 2016 USD ($) Claim	Dec. 31, 2015 USD ($)	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 ($)	Dec. 31, 2007 USD ($)
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 787
IBNR	319
2007 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 50	$ 49	$ 49	$ 51	$ 56	$ 59	$ 57	$ 81	$ 107	$ 98
Cumulative number of claims | Claim	6,270
2008 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 44	45	48	52	61	68	73	114	$ 114
Cumulative number of claims | Claim	7,153
2009 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 53	52	59	68	85	103	114	$ 114
IBNR	$ 1
Cumulative number of claims | Claim	6,654
2010 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 63	66	76	84	111	112	$ 112
IBNR	$ 8
Cumulative number of claims | Claim	5,943
2011 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 70	75	87	116	121	$ 120
IBNR	$ 9
Cumulative number of claims | Claim	5,760
2012 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 52	70	98	122	$ 120
IBNR	$ 16
Cumulative number of claims | Claim	5,473
2013 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 106	115	121	$ 120
IBNR	$ 24
Cumulative number of claims | Claim	4,890
2014 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 94	124	$ 123
IBNR	$ 51
Cumulative number of claims | Claim	4,737
2015 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 131	$ 131
IBNR	$ 100
Cumulative number of claims | Claim	4,279
2016 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 124
IBNR	$ 110
Cumulative number of claims | Claim	2,902