Claim, Claim Adjustment Expense and Future Policy Benefit Reserves - Schedule of Net Cumulative Paid Claim and Claim Adjustment Expenses (Workers' Compensation) (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] | Workers' Compensation [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	3,319
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	1,868
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2010	2,061
Other	(22)
Liability for unallocated claim adjustment expenses for accident years presented	25
Total net liability for unpaid claim and claim adjustment expenses	3,932
Property and Casualty Operations [Member] | Workers' Compensation [Member] | 2010 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	631	625	615	577	$ 542	$ 510	$ 442	$ 359	$ 251	$ 97
Property and Casualty Operations [Member] | Workers' Compensation [Member] | 2011 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	581	571	564	522	478	438	358	249	$ 99
Property and Casualty Operations [Member] | Workers' Compensation [Member] | 2012 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	536	524	509	470	416	342	232	$ 87
Property and Casualty Operations [Member] | Workers' Compensation [Member] | 2013 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	411	419	417	370	300	213	$ 80
Property and Casualty Operations [Member] | Workers' Compensation [Member] | 2014 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	290	282	258	215	159	$ 61
Property and Casualty Operations [Member] | Workers' Compensation [Member] | 2015 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	231	212	180	131	$ 51
Property and Casualty Operations [Member] | Workers' Compensation [Member] | 2016 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	198	169	129	$ 53
Property and Casualty Operations [Member] | Workers' Compensation [Member] | 2017 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	207	151	$ 63
Property and Casualty Operations [Member] | Workers' Compensation [Member] | 2018 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	163	$ 68
Property and Casualty Operations [Member] | Workers' Compensation [Member] | 2019 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 71