Claim, Claim Adjustment Expense and Future Policy Benefit Reserves - Schedule of Net Incurred Claims and Allocated Claim Adjustment Expenses (General Liability) (Detail) - Property and Casualty Operations [Member] - General Liability [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	$ 6,492
IBNR	1,629
2010 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	690	$ 691	$ 691	$ 697	$ 726	$ 750	$ 709	$ 658	$ 664	$ 646
IBNR	$ 19
Cumulative number of claims | Claim	44,229
2011 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 667	669	670	681	676	677	631	589	$ 591
IBNR	$ 20
Cumulative number of claims | Claim	39,361
2012 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 630	635	635	619	636	639	611	$ 587
IBNR	$ 31
Cumulative number of claims | Claim	35,219
2013 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 620	623	613	655	650	655	$ 650
IBNR	$ 27
Cumulative number of claims | Claim	33,570
2014 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 658	635	631	654	658	$ 653
IBNR	$ 57
Cumulative number of claims | Claim	27,877
2015 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 600	589	574	576	$ 581
IBNR	$ 73
Cumulative number of claims | Claim	23,834
2016 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 671	667	659	$ 623
IBNR	$ 166
Cumulative number of claims | Claim	23,817
2017 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 632	632	$ 632
IBNR	$ 226
Cumulative number of claims | Claim	21,114
2018 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 644	$ 653
IBNR	$ 408
Cumulative number of claims | Claim	17,889
2019 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 680
IBNR	$ 602
Cumulative number of claims | Claim	12,916