Claim, Claim Adjustment Expense and Future Policy Benefit Reserves - Schedule of Net Cumulative Paid Claim and Claim Adjustment Expenses (Surety) (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] | Surety [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	427
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	339
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2010	10
Liability for unallocated claim adjustment expenses for accident years presented	20
Total net liability for unpaid claim and claim adjustment expenses	369
Property and Casualty Operations [Member] | Surety [Member] | 2010 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	53	52	52	55	$ 58	$ 57	$ 55	$ 50	$ 34	$ 13
Property and Casualty Operations [Member] | Surety [Member] | 2011 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	57	57	56	60	60	58	55	42	$ 19
Property and Casualty Operations [Member] | Surety [Member] | 2012 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	37	37	36	35	35	34	32	$ 5
Property and Casualty Operations [Member] | Surety [Member] | 2013 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	77	78	78	78	69	40	$ 16
Property and Casualty Operations [Member] | Surety [Member] | 2014 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	38	38	36	38	30	$ 7
Property and Casualty Operations [Member] | Surety [Member] | 2015 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	42	40	38	26	$ 7
Property and Casualty Operations [Member] | Surety [Member] | 2016 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	45	45	37	$ 5
Property and Casualty Operations [Member] | Surety [Member] | 2017 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	41	37	$ 23
Property and Casualty Operations [Member] | Surety [Member] | 2018 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	25	$ 5
Property and Casualty Operations [Member] | Surety [Member] | 2019 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 12