Claim and Claim Adjustment Expense Reserves - Schedule of Net Cumulative Paid Claim and Claim Adjustment Expenses (Medical Professional Liability) (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] | Medical Professional Liability [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	3,156
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	1,716
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2007	30
Liability for unallocated claim adjustment expenses for accident years presented	33
Total net liability for unpaid claim and claim adjustment expenses	1,779
Property and Casualty Operations [Member] | Medical Professional Liability [Member] | 2007 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	365	364	352	326	$ 296	$ 247	$ 201	$ 134	$ 68	$ 11
Property and Casualty Operations [Member] | Medical Professional Liability [Member] | 2008 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	428	409	395	377	332	282	207	90	$ 9
Property and Casualty Operations [Member] | Medical Professional Liability [Member] | 2009 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	396	377	353	328	278	180	75	$ 9
Property and Casualty Operations [Member] | Medical Professional Liability [Member] | 2010 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	371	361	338	273	186	93	$ 11
Property and Casualty Operations [Member] | Medical Professional Liability [Member] | 2011 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	407	379	315	225	121	$ 18
Property and Casualty Operations [Member] | Medical Professional Liability [Member] | 2012 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	428	359	236	121	$ 15
Property and Casualty Operations [Member] | Medical Professional Liability [Member] | 2013 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	364	259	121	$ 18
Property and Casualty Operations [Member] | Medical Professional Liability [Member] | 2014 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	274	149	$ 25
Property and Casualty Operations [Member] | Medical Professional Liability [Member] | 2015 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	105	$ 22
Property and Casualty Operations [Member] | Medical Professional Liability [Member] | 2016 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 18