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, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008	Dec. 31, 2007
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 2007	$ 4,167	$ 4,089	$ 4,355
Total net liability for unpaid claim and claim adjustment expenses	18,249	18,576	18,927	$ 19,117
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	14,910	15,363
Property and Casualty Operations [Member] | Workers' Compensation [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	3,877
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	1,998
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2007	2,334
Other	(30)
Liability for unallocated claim adjustment expenses for accident years presented	4
Total net liability for unpaid claim and claim adjustment expenses	4,306
Property and Casualty Operations [Member] | Workers' Compensation [Member] | 2007 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	535	533	522	502	$ 471	$ 429	$ 390	$ 337	$ 246	$ 100
Property and Casualty Operations [Member] | Workers' Compensation [Member] | 2008 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	520	505	489	461	425	381	323	233	$ 92
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	516	495	468	435	381	315	223	$ 88
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	565	531	500	433	352	245	$ 94
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	515	471	432	353	245	$ 97
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	465	411	338	229	$ 86
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	366	297	211	$ 79
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	213	157	$ 60
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	130	$ 50
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	$ 52