Claim and Claim Adjustment Expense Reserves - Additional Information (Detail) - CNA Financial [Member] - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2010	Dec. 31, 2016
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Catastrophe losses, net of reinsurance	$ 269	$ 16	$ 342	$ 137
Catastrophe-related reinsurance reinstatement premium	6		6
Net A&EP adverse development before consideration of LPT			60	$ 200
Fair value of the collateral trust account	2,900		2,900			$ 2,800
Hurricane Harvey [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Catastrophe losses, net of reinsurance	149		149
Hurricane Irma [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Catastrophe losses, net of reinsurance	95		95
Hurricane Maria [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Catastrophe losses, net of reinsurance	20		20
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	2,900		2,900			2,800
Remaining unrecognized retroactive reinsurance benefit	$ 334		$ 334			$ 334