Claim and Claim Adjustment Expense Reserves - Schedule of Net Cumulative Paid Claim and Claim Adjustment Expenses (Commercial Auto) (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] | Commercial Auto [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	2,220
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	410
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2007	4
Liability for unallocated claim adjustment expenses for accident years presented	10
Total net liability for unpaid claim and claim adjustment expenses	424
Property and Casualty Operations [Member] | Commercial Auto [Member] | 2007 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	351	350	349	348	$ 340	$ 329	$ 295	$ 250	$ 185	$ 93
Property and Casualty Operations [Member] | Commercial Auto [Member] | 2008 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	293	292	291	289	274	244	210	158	$ 83
Property and Casualty Operations [Member] | Commercial Auto [Member] | 2009 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	270	270	269	257	229	188	128	$ 72
Property and Casualty Operations [Member] | Commercial Auto [Member] | 2010 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	279	274	265	240	197	137	$ 72
Property and Casualty Operations [Member] | Commercial Auto [Member] | 2011 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	275	264	241	193	141	$ 78
Property and Casualty Operations [Member] | Commercial Auto [Member] | 2012 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	276	253	214	157	$ 77
Property and Casualty Operations [Member] | Commercial Auto [Member] | 2013 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	195	164	132	$ 73
Property and Casualty Operations [Member] | Commercial Auto [Member] | 2014 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	135	100	$ 63
Property and Casualty Operations [Member] | Commercial Auto [Member] | 2015 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	95	$ 52
Property and Casualty Operations [Member] | Commercial Auto [Member] | 2016 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 51