Claim, Claim Adjustment Expense and Future Policy Benefit Reserves - Schedule of Net Incurred Claims and Allocated Claim Adjustment Expenses (Commercial Auto) (Detail) - Property and Casualty Operations [Member] - Commercial Auto [Member] - CNA Financial Corporation [Member] $ in Millions	Dec. 31, 2019 USD ($) Claim	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 2,371
IBNR	207
2010 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	288	$ 288	$ 288	$ 289	$ 293	$ 298	$ 291	$ 287	$ 283	$ 267
IBNR	$ 1
Cumulative number of claims | Claim	48,035
2011 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 291	294	294	294	300	302	288	281	$ 268
Cumulative number of claims | Claim	47,909
2012 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 297	299	299	307	303	299	289	$ 275
IBNR	$ 3
Cumulative number of claims | Claim	46,288
2013 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 241	245	245	249	265	265	$ 246
IBNR	$ 2
Cumulative number of claims | Claim	39,429
2014 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 201	205	205	212	223	$ 234
IBNR	$ 3
Cumulative number of claims | Claim	33,622
2015 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 183	190	190	199	$ 201
IBNR	$ 7
Cumulative number of claims | Claim	30,418
2016 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 186	186	186	$ 198
IBNR	$ 7
Cumulative number of claims | Claim	30,414
2017 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 200	198	$ 199
IBNR	$ 9
Cumulative number of claims | Claim	30,850
2018 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 227	$ 229
IBNR	$ 47
Cumulative number of claims | Claim	33,959
2019 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 257
IBNR	$ 128
Cumulative number of claims | Claim	31,455