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, 2018 USD ($) Claim	Dec. 31, 2017 USD ($)	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 ($)	Dec. 31, 2009 USD ($)
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 2,411
IBNR	239
2009 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 277	$ 276	$ 276	$ 278	$ 281	$ 285	$ 282	$ 280	$ 276	$ 291
Cumulative number of claims | Claim	48,499
2010 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 288	288	289	293	298	291	287	283	$ 267
IBNR	$ 1
Cumulative number of claims | Claim	48,030
2011 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 294	294	294	300	302	288	281	$ 268
IBNR	$ 4
Cumulative number of claims | Claim	47,905
2012 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 299	299	307	303	299	289	$ 275
IBNR	$ 6
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	$ 245	245	249	265	265	$ 246
IBNR	$ 7
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	$ 205	205	212	223	$ 234
IBNR	$ 9
Cumulative number of claims | Claim	33,609
2015 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 190	190	199	$ 201
IBNR	$ 22
Cumulative number of claims | Claim	30,388
2016 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 186	186	$ 198
IBNR	$ 27
Cumulative number of claims | Claim	30,342
2017 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 198	$ 199
IBNR	$ 45
Cumulative number of claims | Claim	30,580
2018 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 229
IBNR	$ 118
Cumulative number of claims | Claim	28,602