Claim, Claim Adjustment Expense and Future Policy Benefit Reserves - Schedule of Net Incurred Claims and Allocated Claim Adjustment Expenses (Surety) (Detail) - Property and Casualty Operations [Member] - Surety [Member] - CNA Financial Corporation [Member] Claim in Millions, $ in Millions	Dec. 31, 2019 USD ($) Claim	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 766
IBNR	289
2010 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 61	$ 61	$ 59	$ 63	$ 66	$ 76	$ 84	$ 111	$ 112	$ 112
Cumulative number of claims | Claim	5,982
2011 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 62	62	66	70	75	87	116	121	$ 120
IBNR	$ 2
Cumulative number of claims | Claim	5,813
2012 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 38	39	45	52	70	98	122	$ 120
IBNR	$ 1
Cumulative number of claims | Claim	5,568
2013 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 83	87	91	106	115	121	$ 120
IBNR	$ 3
Cumulative number of claims | Claim	5,062
2014 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 45	60	69	94	124	$ 123
IBNR	$ 4
Cumulative number of claims | Claim	5,078
2015 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 63	79	104	131	$ 131
IBNR	$ 11
Cumulative number of claims | Claim	4,976
2016 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 84	109	124	$ 124
IBNR	$ 36
Cumulative number of claims | Claim	5,379
2017 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 103	115	$ 120
IBNR	$ 54
Cumulative number of claims | Claim	5,496
2018 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 108	$ 114
IBNR	$ 76
Cumulative number of claims | Claim	5,451
2019 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 119
IBNR	$ 102
Cumulative number of claims | Claim	3,549