Claim and Claim Adjustment Expense Reserves - Schedule of Net Cumulative Paid Claim and Claim Adjustment Expenses (Workers' Compensation) (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] | Workers' Compensation [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	3,578
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	1,867
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2009	2,131
Other	(32)
Liability for unallocated claim adjustment expenses for accident years presented	2
Total net liability for unpaid claim and claim adjustment expenses	3,968
Property and Casualty Operations [Member] | Workers' Compensation [Member] | 2009 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	557	549	525	503	$ 476	$ 443	$ 388	$ 321	$ 227	$ 89
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	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	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	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	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	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	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	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	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	$ 68