Claim, Claim Adjustment Expense and Future Policy Benefit Reserves - Additional Information (Detail) - CNA Financial Corporation [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2010
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net A&EP adverse development before consideration of LPT	$ 150.0	$ 178.0	$ 60.0
Fair value of the collateral trust account	3,700.0	2,700.0
Changes In Claims And Claims Adjustment Reserve	56.0	31.0
Additions To Long-Duration Insurance at Original Discount Rate	216.0
Provision for uncollectible third-party reinsurance on A&EP	(25.0)	(16.0)
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				$ 1,600.0
Aggregate limit under A&EP Loss Portfolio Transfer				4.0
Ceded A&EP claim and allocated claim adjustment expense reserves				1,200.0
Reinsurance premium paid to NICO under A&EP Loss Portfolio Transfer				2,000.0
Net reinsurance receivables transferred to NICO under A&EP Loss Portfolio Transfer				215.0
Total consideration				$ 2,200.0
Cumulative amounts ceded under the Loss Portfolio Transfer	3,200.0	3,100.0
Remaining unrecognized retroactive reinsurance benefit	$ 392.0	$ 374.0