Claim, Claim Adjustment Expense and Future Policy Benefit Reserves - Schedule of Net Cumulative Paid Claim and Claim Adjustment Expenses (Commercial Auto) (Detail) - CNA Financial Corporation [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2010	$ 4,584	$ 4,285	$ 4,366
Total net liability for unpaid claim and claim adjustment expenses	17,885	17,965	18,070	$ 18,249
Property and Casualty Operations [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	14,153	14,353
Property and Casualty Operations [Member] | Commercial Auto [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	1,982
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	389
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2010	1
Liability for unallocated claim adjustment expenses for accident years presented	14
Total net liability for unpaid claim and claim adjustment expenses	404
Property and Casualty Operations [Member] | Commercial Auto [Member] | 2010 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	287	287	287	286	$ 281	$ 271	$ 246	$ 203	$ 141	$ 74
Property and Casualty Operations [Member] | Commercial Auto [Member] | 2011 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	289	289	287	284	274	248	199	145	$ 79
Property and Casualty Operations [Member] | Commercial Auto [Member] | 2012 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	291	290	285	282	259	220	160	$ 78
Property and Casualty Operations [Member] | Commercial Auto [Member] | 2013 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	238	234	225	200	168	135	$ 74
Property and Casualty Operations [Member] | Commercial Auto [Member] | 2014 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	196	187	166	137	102	$ 64
Property and Casualty Operations [Member] | Commercial Auto [Member] | 2015 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	172	153	130	96	$ 52
Property and Casualty Operations [Member] | Commercial Auto [Member] | 2016 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	154	126	93	$ 52
Property and Casualty Operations [Member] | Commercial Auto [Member] | 2017 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	150	107	$ 58
Property and Casualty Operations [Member] | Commercial Auto [Member] | 2018 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	128	$ 66
Property and Casualty Operations [Member] | Commercial Auto [Member] | 2019 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 77