Claim and Claim Adjustment Expense Reserves - Schedule of Net Cumulative Paid Claim and Claim Adjustment Expenses (Other Professional Liability and Management Liability) (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] | Other Professional Liability and Management Liability [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	5,864
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	2,695
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2009	90
Liability for unallocated claim adjustment expenses for accident years presented	64
Total net liability for unpaid claim and claim adjustment expenses	2,849
Property and Casualty Operations [Member] | Other Professional Liability and Management Liability [Member] | 2009 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	844	825	801	772	$ 722	$ 641	$ 553	$ 361	$ 195	$ 37
Property and Casualty Operations [Member] | Other Professional Liability and Management Liability [Member] | 2010 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	784	752	721	670	630	541	405	204	$ 31
Property and Casualty Operations [Member] | Other Professional Liability and Management Liability [Member] | 2011 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	796	781	726	683	605	503	314	$ 71
Property and Casualty Operations [Member] | Other Professional Liability and Management Liability [Member] | 2012 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	755	711	651	573	400	248	$ 56
Property and Casualty Operations [Member] | Other Professional Liability and Management Liability [Member] | 2013 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	754	702	618	447	249	$ 54
Property and Casualty Operations [Member] | Other Professional Liability and Management Liability [Member] | 2014 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	647	515	392	223	$ 51
Property and Casualty Operations [Member] | Other Professional Liability and Management Liability [Member] | 2015 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	542	404	234	$ 60
Property and Casualty Operations [Member] | Other Professional Liability and Management Liability [Member] | 2016 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	466	248	$ 64
Property and Casualty Operations [Member] | Other Professional Liability and Management Liability [Member] | 2017 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	222	$ 57
Property and Casualty Operations [Member] | Other Professional Liability and Management Liability [Member] | 2018 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 54