Claim and Claim Adjustment Expense Reserves - Reconciliation of Claim and Claim Adjustment Expense Reserves (Detail) - CNA Financial [Member] - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross reserves, beginning of year	$ 22,004	$ 22,343
Ceded reserves, beginning of year	3,934	4,094
Net reserves, beginning of year	18,070	18,249
Net incurred claim and claim adjustment expenses:
Provision for insured events of current year	2,552	2,443
Decrease in provision for insured events of prior years	(112)	(159)
Amortization of discount	92	93
Total net incurred	2,532	2,377
Net payments attributable to:
Current year events	(312)	(266)
Prior year events	(2,387)	(2,331)
Total net payments	(2,699)	(2,597)
Foreign currency translation adjustment and other	(70)	70
Net reserves, end of period	17,833	18,099
Ceded reserves, end of period	4,157	4,080
Gross reserves, end of period	$ 21,990	$ 22,179