Claim and Claim Adjustment Expense Reserves - Reconciliation of Claim and Claim Adjustment Expense Reserves (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net incurred claim and claim adjustment expenses:
Provision for insured events of current year	$ 5,358	$ 5,201	$ 5,025
Decrease in provision for insured events of prior years	(179)	(381)	(342)
CNA Financial [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross reserves, beginning of year	22,004	22,343	22,663
Ceded reserves, beginning of year	3,934	4,094	4,087
Net reserves, beginning of year	18,070	18,249	18,576
Net incurred claim and claim adjustment expenses:
Provision for insured events of current year	5,358	5,201	5,025
Decrease in provision for insured events of prior years	(179)	(381)	(342)
Amortization of discount	176	179	175
Total net incurred	5,355	4,999	4,858
Net payments attributable to:
Current year events	(1,046)	(975)	(967)
Prior year events	(4,285)	(4,366)	(4,167)
Total net payments	(5,331)	(5,341)	(5,134)
Foreign currency translation adjustment and other	(129)	163	(51)
Net reserves, end of year	17,965	18,070	18,249
Ceded reserves, end of year	4,019	3,934	4,094
Gross reserves, end of year	$ 21,984	$ 22,004	$ 22,343