Claim, Claim Adjustment Expense and Future Policy Benefit Reserves - Schedule of Net Incurred Claims and Allocated Claim Adjustment Expenses (Medical Professional Liability) (Detail) - Property and Casualty Operations [Member] - Medical Professional Liability [Member] - CNA Financial Corporation [Member] Claim in Millions, $ in Millions	Dec. 31, 2019 USD ($) Claim	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 4,685
IBNR	853
2010 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	369	$ 356	$ 360	$ 365	$ 395	$ 415	$ 426	$ 423	$ 412	$ 402
IBNR	$ 1
Cumulative number of claims | Claim	14,624
2011 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 439	437	437	434	439	468	443	437	$ 429
IBNR	$ 2
Cumulative number of claims | Claim	16,526
2012 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 499	493	484	493	498	508	469	$ 464
IBNR	$ 8
Cumulative number of claims | Claim	17,724
2013 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 545	535	525	513	500	479	$ 462
IBNR	$ 27
Cumulative number of claims | Claim	19,510
2014 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 529	535	530	537	489	$ 450
IBNR	$ 16
Cumulative number of claims | Claim	19,723
2015 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 488	494	510	499	$ 433
IBNR	$ 29
Cumulative number of claims | Claim	18,029
2016 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 499	485	487	$ 427
IBNR	$ 63
Cumulative number of claims | Claim	15,823
2017 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 458	449	$ 412
IBNR	$ 127
Cumulative number of claims | Claim	14,636
2018 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 429	$ 404
IBNR	$ 216
Cumulative number of claims | Claim	13,760
2019 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 430
IBNR	$ 364
Cumulative number of claims | Claim	10,467