Claim and Claim Adjustment Expense Reserves - Schedule of Net Cumulative Paid Claim and Claim Adjustment Expenses (Other Professional Liability and Management Liability) (Detail) - CNA Financial [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008	Dec. 31, 2007
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2007	$ 4,167	$ 4,089	$ 4,355
Total net liability for unpaid claim and claim adjustment expenses	18,249	18,576	18,927	$ 19,117
Property and Casualty Operations [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	14,910	15,363
Property and Casualty Operations [Member] | Other Professional Liability and Management Liability [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	5,475
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	2,887
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2007	104
Liability for unallocated claim adjustment expenses for accident years presented	72
Total net liability for unpaid claim and claim adjustment expenses	3,063
Property and Casualty Operations [Member] | Other Professional Liability and Management Liability [Member] | 2007 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	614	593	585	564	$ 524	$ 472	$ 397	$ 307	$ 162	$ 32
Property and Casualty Operations [Member] | Other Professional Liability and Management Liability [Member] | 2008 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	741	719	678	641	600	515	376	181	$ 39
Property and Casualty Operations [Member] | Other Professional Liability and Management Liability [Member] | 2009 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	798	769	719	638	550	358	195	$ 37
Property and Casualty Operations [Member] | Other Professional Liability and Management Liability [Member] | 2010 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	721	670	630	541	404	203	$ 31
Property and Casualty Operations [Member] | Other Professional Liability and Management Liability [Member] | 2011 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	726	682	604	502	313	$ 71
Property and Casualty Operations [Member] | Other Professional Liability and Management Liability [Member] | 2012 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	648	570	398	248	$ 57
Property and Casualty Operations [Member] | Other Professional Liability and Management Liability [Member] | 2013 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	583	426	240	$ 51
Property and Casualty Operations [Member] | Other Professional Liability and Management Liability [Member] | 2014 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	375	212	$ 51
Property and Casualty Operations [Member] | Other Professional Liability and Management Liability [Member] | 2015 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	209	$ 48
Property and Casualty Operations [Member] | Other Professional Liability and Management Liability [Member] | 2016 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 60