Claim, Claim Adjustment Expense and Future Policy Benefit Reserves - Schedule of Net Incurred Claims and Allocated Claim Adjustment Expenses (Other Professional Liability and Management Liability) (Detail) - Property and Casualty Operations [Member] - Other Professional Liability and Management Liability [Member] - CNA Financial Corporation [Member] $ 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	$ 8,466
IBNR	1,957
2010 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	821	$ 821	$ 827	$ 824	$ 837	$ 847	$ 848	$ 848	$ 828	$ 828
IBNR	$ 9
Cumulative number of claims | Claim	17,891
2011 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 885	888	899	911	944	949	934	908	$ 880
IBNR	$ 21
Cumulative number of claims | Claim	18,738
2012 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 831	833	846	840	878	887	909	$ 923
IBNR	$ 18
Cumulative number of claims | Claim	18,499
2013 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 850	863	866	885	926	894	$ 884
IBNR	$ 45
Cumulative number of claims | Claim	17,928
2014 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 854	835	831	885	898	$ 878
IBNR	$ 74
Cumulative number of claims | Claim	17,553
2015 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 807	832	877	892	$ 888
IBNR	$ 120
Cumulative number of claims | Claim	17,390
2016 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 904	900	900	$ 901
IBNR	$ 188
Cumulative number of claims | Claim	17,890
2017 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 813	845	$ 847
IBNR	$ 308
Cumulative number of claims | Claim	18,015
2018 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 864	$ 850
IBNR	$ 460
Cumulative number of claims | Claim	19,468
2019 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 837
IBNR	$ 714
Cumulative number of claims | Claim	16,722