Claim and Claim Adjustment Expense Reserves - Additional Information (Detail) - CNA Financial [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2010
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net unfavorable prior year development recognized before consideration of LPT	$ 178	$ 60	$ 200
Fair value of the collateral trust account	2,700	3,100
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	2,900
Remaining unrecognized retroactive reinsurance benefit	374	326
Other Insurance Operations [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Favorable net prior year development	$ 19	$ 72	$ 46