Claim and Claim Adjustment Expense Reserves - Schedule of Net Cumulative Paid Claim and Claim Adjustment Expenses (General Liability) (Detail) - CNA Financial [Member] - USD ($) $ in Millions	12 Months Ended
	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	Dec. 31, 2009
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 2009	$ 4,285	$ 4,366	$ 4,167
Total net liability for unpaid claim and claim adjustment expenses	17,965	18,070	18,249	$ 18,576
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	14,353	14,575
Property and Casualty Operations [Member] | General Liability [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	4,342
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	2,207
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2009	927
Liability for unallocated claim adjustment expenses for accident years presented	61
Total net liability for unpaid claim and claim adjustment expenses	3,195
Property and Casualty Operations [Member] | General Liability [Member] | 2009 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	727	721	701	663	$ 625	$ 576	$ 468	$ 305	$ 124	$ 33
Property and Casualty Operations [Member] | General Liability [Member] | 2010 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	656	652	642	611	561	429	280	145	$ 27
Property and Casualty Operations [Member] | General Liability [Member] | 2011 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	622	602	568	517	411	273	148	$ 28
Property and Casualty Operations [Member] | General Liability [Member] | 2012 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	559	510	454	374	247	132	$ 28
Property and Casualty Operations [Member] | General Liability [Member] | 2013 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	510	450	352	240	128	$ 31
Property and Casualty Operations [Member] | General Liability [Member] | 2014 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	481	376	247	119	$ 31
Property and Casualty Operations [Member] | General Liability [Member] | 2015 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	357	230	110	$ 19
Property and Casualty Operations [Member] | General Liability [Member] | 2016 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	279	163	$ 32
Property and Casualty Operations [Member] | General Liability [Member] | 2017 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	118	$ 23
Property and Casualty Operations [Member] | General Liability [Member] | 2018 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 33