Claim and Claim Adjustment Expense Reserves - Additional Information (Detail) - CNA Financial [Member] - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2010	Dec. 31, 2018
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Catastrophe losses, net of reinsurance	$ 38	$ 26	$ 96	$ 60
Net A&EP adverse development before consideration of LPT				113
Fair value of the collateral trust account	3,100		3,100			$ 2,700
Asbestos and Environmental Pollution Reserves [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net A&EP claim and allocated claim adjustment expense reserves					$ 1,600
Aggregate limit under A&EP Loss Portfolio Transfer					4,000
Ceded A&EP claim and allocated claim adjustment expense reserves					1,200
Reinsurance premium paid to NICO under A&EP Loss Portfolio Transfer					2,000
Net reinsurance receivables transferred to NICO under A&EP Loss Portfolio Transfer					215
Total consideration					$ 2,200
Cumulative amounts ceded under the Loss Portfolio Transfer	3,100		3,100			3,100
Remaining unrecognized retroactive reinsurance benefit	338		338			$ 374
Property and Casualty Operations [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Favorable net prior year development	$ 31	$ 59	$ 45	$ 98