Claim and Claim Adjustment Expense Reserves - Schedule of Net Incurred Claims and Allocated Claim Adjustment Expenses (Commercial Auto) (Detail) - Property and Casualty Operations [Member] - Commercial Auto [Member] - CNA Financial [Member] $ in Millions	Dec. 31, 2016 USD ($) Claim	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)	Dec. 31, 2009 USD ($)	Dec. 31, 2008 USD ($)	Dec. 31, 2007 USD ($)
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 2,630
IBNR	247
2007 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 352	$ 354	$ 355	$ 356	$ 358	$ 355	$ 360	$ 368	$ 367	$ 348
Cumulative number of claims | Claim	67,473
2008 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 296	298	298	305	309	306	316	323	$ 322
Cumulative number of claims | Claim	56,407
2009 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 272	275	277	281	278	274	272	$ 287
Cumulative number of claims | Claim	47,325
2010 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 281	286	291	283	279	274	$ 262
IBNR	$ 1
Cumulative number of claims | Claim	46,324
2011 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 285	290	293	279	273	$ 262
IBNR	$ 5
Cumulative number of claims | Claim	46,676
2012 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 300	296	292	282	$ 270
IBNR	$ 11
Cumulative number of claims | Claim	45,279
2013 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 241	257	259	$ 242
IBNR	$ 20
Cumulative number of claims | Claim	38,513
2014 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 210	221	$ 231
IBNR	$ 40
Cumulative number of claims | Claim	32,958
2015 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 197	$ 199
IBNR	$ 65
Cumulative number of claims | Claim	29,714
2016 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 196
IBNR	$ 105
Cumulative number of claims | Claim	25,196