Claim and Claim Adjustment Expense Reserves - Schedule of Net Cumulative Paid Claim and Claim Adjustment Expenses (Commercial Auto) (Detail) - CNA Financial [Member] - USD ($) $ in Millions	12 Months Ended
	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	Dec. 31, 2009
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 2009	$ 4,285	$ 4,366	$ 4,167
Total net liability for unpaid claim and claim adjustment expenses	17,965	18,070	18,249	$ 18,576
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	14,353	14,575
Property and Casualty Operations [Member] | Commercial Auto [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	2,016
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	395
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2009	6
Liability for unallocated claim adjustment expenses for accident years presented	11
Total net liability for unpaid claim and claim adjustment expenses	412
Property and Casualty Operations [Member] | Commercial Auto [Member] | 2009 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	277	275	274	274	$ 272	$ 261	$ 233	$ 191	$ 130	$ 73
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	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	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	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	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	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	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	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	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	$ 66