Claim and Claim Adjustment Expense Reserves - Schedule of Net Incurred Claims and Allocated Claim Adjustment Expenses (Workers' Compensation) (Detail) - Property and Casualty Operations [Member] - Workers' Compensation [Member] - CNA Financial [Member] $ in Millions	Dec. 31, 2018 USD ($) Claim	Dec. 31, 2017 USD ($)	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 ($)	Dec. 31, 2009 USD ($)
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 5,445
IBNR	1,130
2009 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	650	$ 649	$ 638	$ 631	$ 626	$ 616	$ 611	$ 609	$ 599	$ 592
IBNR	$ 46
Cumulative number of claims | Claim	51,822
2010 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 732	733	730	710	698	676	654	632	$ 583
IBNR	$ 44
Cumulative number of claims | Claim	49,106
2011 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 674	676	676	651	659	647	641	$ 607
IBNR	$ 27
Cumulative number of claims | Claim	45,637
2012 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 671	673	678	669	659	627	$ 601
IBNR	$ 59
Cumulative number of claims | Claim	42,477
2013 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 582	593	618	592	572	$ 537
IBNR	$ 86
Cumulative number of claims | Claim	38,665
2014 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 450	452	479	480	$ 467
IBNR	$ 104
Cumulative number of claims | Claim	33,465
2015 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 408	406	431	$ 422
IBNR	$ 146
Cumulative number of claims | Claim	31,828
2016 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 396	405	$ 426
IBNR	$ 171
Cumulative number of claims | Claim	31,905
2017 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 432	$ 440
IBNR	$ 171
Cumulative number of claims | Claim	32,811
2018 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 450
IBNR	$ 276
Cumulative number of claims | Claim	30,399