Claim, Claim Adjustment Expense and Future Policy Benefit Reserves - Schedule of Net Cumulative Paid Claim and Claim Adjustment Expenses (Medical Professional Liability) (Detail) - CNA Financial Corporation [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2010	$ 4,584	$ 4,285	$ 4,366
Total net liability for unpaid claim and claim adjustment expenses	17,885	17,965	18,070	$ 18,249
Property and Casualty Operations [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	14,153	14,353
Property and Casualty Operations [Member] | Medical Professional Liability [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	3,303
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2010	1,382
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	22
Liability for unallocated claim adjustment expenses for accident years presented	25
Total net liability for unpaid claim and claim adjustment expenses	1,429
Property and Casualty Operations [Member] | Medical Professional Liability [Member] | 2010 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	353	350	346	337	$ 326	$ 306	$ 257	$ 173	$ 86	$ 10
Property and Casualty Operations [Member] | Medical Professional Liability [Member] | 2011 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	414	409	398	375	347	295	208	109	$ 17
Property and Casualty Operations [Member] | Medical Professional Liability [Member] | 2012 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	479	457	427	388	323	221	117	$ 14
Property and Casualty Operations [Member] | Medical Professional Liability [Member] | 2013 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	495	462	414	355	255	119	$ 17
Property and Casualty Operations [Member] | Medical Professional Liability [Member] | 2014 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	472	417	359	258	136	$ 23
Property and Casualty Operations [Member] | Medical Professional Liability [Member] | 2015 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	384	313	230	101	$ 22
Property and Casualty Operations [Member] | Medical Professional Liability [Member] | 2016 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	339	246	121	$ 18
Property and Casualty Operations [Member] | Medical Professional Liability [Member] | 2017 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	235	107	$ 19
Property and Casualty Operations [Member] | Medical Professional Liability [Member] | 2018 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	115	$ 21
Property and Casualty Operations [Member] | Medical Professional Liability [Member] | 2019 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 17