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, 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	$ 6,065
IBNR	1,382
2007 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	540	$ 548	$ 545	$ 559	$ 600	$ 610	$ 639	$ 678	$ 673	$ 774
IBNR	$ 28
Cumulative number of claims | Claim	53,553
2008 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 591	600	613	632	633	647	630	604	$ 611
IBNR	$ 18
Cumulative number of claims | Claim	44,586
2009 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 622	619	623	629	633	634	637	$ 591
IBNR	$ 16
Cumulative number of claims | Claim	43,955
2010 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 659	675	695	649	599	597	$ 566
IBNR	$ 25
Cumulative number of claims | Claim	43,378
2011 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 621	611	610	564	534	$ 537
IBNR	$ 41
Cumulative number of claims | Claim	38,101
2012 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 558	570	579	563	$ 539
IBNR	$ 63
Cumulative number of claims | Claim	34,037
2013 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 643	634	645	$ 615
IBNR	$ 142
Cumulative number of claims | Claim	32,897
2014 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 635	634	$ 627
IBNR	$ 224
Cumulative number of claims | Claim	26,744
2015 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 574	$ 573
IBNR	$ 330
Cumulative number of claims | Claim	21,687
2016 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 622
IBNR	$ 495
Cumulative number of claims | Claim	16,720