Claim, Claim Adjustment Expense and Future Policy Benefit Reserves - Schedule of Net Cumulative Paid Claim and Claim Adjustment Expenses (Other Professional Liability and Management Liability) (Detail) - 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
CNA Financial Corporation [Member]
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] | CNA Financial Corporation [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] | Other Professional Liability and Management Liability [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	5,865
Property and Casualty Operations [Member] | Other Professional Liability and Management Liability [Member] | CNA Financial Corporation [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	2,601
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2010	88
Liability for unallocated claim adjustment expenses for accident years presented	50
Total net liability for unpaid claim and claim adjustment expenses	2,739
Property and Casualty Operations [Member] | Other Professional Liability and Management Liability [Member] | 2010 [Member] | CNA Financial Corporation [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	790	784	752	721	$ 670	$ 630	$ 541	$ 405	$ 204	$ 31
Property and Casualty Operations [Member] | Other Professional Liability and Management Liability [Member] | 2011 [Member] | CNA Financial Corporation [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	828	796	781	726	683	605	503	314	$ 71
Property and Casualty Operations [Member] | Other Professional Liability and Management Liability [Member] | 2012 [Member] | CNA Financial Corporation [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	792	755	711	651	573	400	248	$ 56
Property and Casualty Operations [Member] | Other Professional Liability and Management Liability [Member] | 2013 [Member] | CNA Financial Corporation [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	771	754	702	618	447	249	$ 54
Property and Casualty Operations [Member] | Other Professional Liability and Management Liability [Member] | 2014 [Member] | CNA Financial Corporation [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	707	647	515	392	223	$ 51
Property and Casualty Operations [Member] | Other Professional Liability and Management Liability [Member] | 2015 [Member] | CNA Financial Corporation [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	612	542	404	234	$ 60
Property and Casualty Operations [Member] | Other Professional Liability and Management Liability [Member] | 2016 [Member] | CNA Financial Corporation [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	625	466	248	$ 64
Property and Casualty Operations [Member] | Other Professional Liability and Management Liability [Member] | 2017 [Member] | CNA Financial Corporation [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	394	222	$ 57
Property and Casualty Operations [Member] | Other Professional Liability and Management Liability [Member] | 2018 [Member] | CNA Financial Corporation [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	282	$ 54
Property and Casualty Operations [Member] | Other Professional Liability and Management Liability [Member] | 2019 [Member] | CNA Financial Corporation [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 64