Claim and Claim Adjustment Expense Reserves - Schedule of Net Incurred Claims and Allocated Claim Adjustment Expenses (Other Professional Liability and Management Liability) (Detail) - Property and Casualty Operations [Member] - Other Professional Liability and Management 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	$ 8,362
IBNR	2,014
2007 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	662	$ 662	$ 681	$ 704	$ 724	$ 734	$ 754	$ 806	$ 817	$ 804
IBNR	$ 13
Cumulative number of claims | Claim	16,011
2008 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 827	845	850	880	915	924	954	933	$ 916
IBNR	$ 35
Cumulative number of claims | Claim	16,326
2009 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 903	895	900	891	898	903	873	$ 829
IBNR	$ 50
Cumulative number of claims | Claim	17,263
2010 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 823	836	846	848	850	827	$ 825
IBNR	$ 39
Cumulative number of claims | Claim	17,796
2011 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 910	944	948	933	904	$ 876
IBNR	$ 107
Cumulative number of claims | Claim	18,620
2012 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 833	870	876	894	$ 907
IBNR	$ 107
Cumulative number of claims | Claim	18,228
2013 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 840	879	841	$ 844
IBNR	$ 137
Cumulative number of claims | Claim	17,324
2014 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 854	859	$ 841
IBNR	$ 306
Cumulative number of claims | Claim	16,886
2015 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 851	$ 847
IBNR	$ 478
Cumulative number of claims | Claim	16,391
2016 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 859
IBNR	$ 742
Cumulative number of claims | Claim	15,045