Claim, Claim Adjustment Expense and Future Policy Benefit Reserves - Schedule of Net Incurred Claims and Allocated Claim Adjustment Expenses (Workers' Compensation) (Detail) - Property and Casualty Operations [Member] - Workers' Compensation [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	$ 5,187
IBNR	1,208
2010 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	735	$ 732	$ 733	$ 730	$ 710	$ 698	$ 676	$ 654	$ 632	$ 583
IBNR	$ 55
Cumulative number of claims | Claim	49,333
2011 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 688	674	676	676	651	659	647	641	$ 607
IBNR	$ 40
Cumulative number of claims | Claim	45,959
2012 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 668	671	673	678	669	659	627	$ 601
IBNR	$ 67
Cumulative number of claims | Claim	42,586
2013 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 561	582	593	618	592	572	$ 537
IBNR	$ 93
Cumulative number of claims | Claim	38,688
2014 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 446	450	452	479	480	$ 467
IBNR	$ 99
Cumulative number of claims | Claim	33,480
2015 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 394	408	406	431	$ 422
IBNR	$ 130
Cumulative number of claims | Claim	31,861
2016 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 382	396	405	$ 426
IBNR	$ 144
Cumulative number of claims | Claim	31,945
2017 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 421	432	$ 440
IBNR	$ 138
Cumulative number of claims | Claim	33,029
2018 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 440	$ 450
IBNR	$ 185
Cumulative number of claims | Claim	34,647
2019 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 452
IBNR	$ 257
Cumulative number of claims | Claim	29,795