Claim and Claim Adjustment Expense Reserves - Schedule of Net Incurred Claims and Allocated Claim Adjustment Expenses (General Liability) (Detail) - Property and Casualty Operations [Member] - General Liability [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	$ 6,549
IBNR	1,586
2009 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	755	$ 755	$ 754	$ 755	$ 756	$ 752	$ 755	$ 733	$ 716	$ 662
IBNR	$ 16
Cumulative number of claims | Claim	44,934
2010 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 691	691	697	726	750	709	658	664	$ 646
IBNR	$ 24
Cumulative number of claims | Claim	44,144
2011 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 669	670	681	676	677	631	589	$ 591
IBNR	$ 26
Cumulative number of claims | Claim	39,283
2012 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 635	635	619	636	639	611	$ 587
IBNR	$ 46
Cumulative number of claims | Claim	35,083
2013 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 623	613	655	650	655	$ 650
IBNR	$ 51
Cumulative number of claims | Claim	33,420
2014 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 635	631	654	658	$ 653
IBNR	$ 71
Cumulative number of claims | Claim	27,736
2015 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 589	574	576	$ 581
IBNR	$ 122
Cumulative number of claims | Claim	23,471
2016 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 667	659	$ 623
IBNR	$ 265
Cumulative number of claims | Claim	23,078
2017 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 632	$ 632
IBNR	$ 412
Cumulative number of claims | Claim	19,716
2018 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 653
IBNR	$ 553
Cumulative number of claims | Claim	13,336