Claim and Claim Adjustment Expense Reserves - Schedule of Net Incurred Claims and Allocated Claim Adjustment Expenses (Workers' Compensation) (Detail) - Property and Casualty Operations [Member] - Workers' Compensation [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	$ 5,875
IBNR	1,068
2007 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	627	$ 608	$ 610	$ 604	$ 603	$ 603	$ 604	$ 596	$ 580	$ 568
IBNR	$ 32
Cumulative number of claims | Claim	71,049
2008 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 638	630	622	612	608	606	593	575	$ 558
IBNR	$ 36
Cumulative number of claims | Claim	59,883
2009 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 625	617	611	600	596	594	587	$ 583
IBNR	$ 46
Cumulative number of claims | Claim	51,111
2010 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 717	697	683	663	641	619	$ 576
IBNR	$ 45
Cumulative number of claims | Claim	48,056
2011 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 666	642	648	637	628	$ 593
IBNR	$ 52
Cumulative number of claims | Claim	44,571
2012 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 671	661	648	616	$ 589
IBNR	$ 86
Cumulative number of claims | Claim	41,683
2013 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 610	584	563	$ 528
IBNR	$ 121
Cumulative number of claims | Claim	38,102
2014 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 474	474	$ 459
IBNR	$ 157
Cumulative number of claims | Claim	32,996
2015 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 426	$ 416
IBNR	$ 206
Cumulative number of claims | Claim	31,296
2016 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 421
IBNR	$ 287
Cumulative number of claims | Claim	27,042