Claim and Claim Adjustment Expense Reserves - Schedule of Net Cumulative Paid Claim and Claim Adjustment Expenses (Medical Professional 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] | Medical Professional Liability [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	3,082
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	1,416
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2009	18
Liability for unallocated claim adjustment expenses for accident years presented	23
Total net liability for unpaid claim and claim adjustment expenses	1,457
Property and Casualty Operations [Member] | Medical Professional Liability [Member] | 2009 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	300	291	288	280	$ 269	$ 247	$ 211	$ 143	$ 63	$ 9
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	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	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	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	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	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	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	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	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	$ 21