Claim and Claim Adjustment Expense 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 [Member] $ in Millions	Dec. 31, 2016 USD ($) Claim	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)	Dec. 31, 2009 USD ($)	Dec. 31, 2008 USD ($)	Dec. 31, 2007 USD ($)
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 4,872
IBNR	1,054
2007 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	385	$ 399	$ 401	$ 390	$ 391	$ 395	$ 427	$ 444	$ 452	$ 448
IBNR	$ 9
Cumulative number of claims | Claim	12,122
2008 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 442	455	467	468	468	480	496	451	$ 426
IBNR	$ 9
Cumulative number of claims | Claim	14,094
2009 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 432	463	471	480	506	494	469	$ 462
IBNR	$ 12
Cumulative number of claims | Claim	15,573
2010 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 403	446	470	486	478	478	$ 483
IBNR	$ 16
Cumulative number of claims | Claim	15,206
2011 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 491	501	533	507	492	$ 486
IBNR	$ 23
Cumulative number of claims | Claim	17,428
2012 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 559	567	575	529	$ 526
IBNR	$ 47
Cumulative number of claims | Claim	18,375
2013 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 567	560	540	$ 534
IBNR	$ 95
Cumulative number of claims | Claim	19,565
2014 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 585	548	$ 511
IBNR	$ 165
Cumulative number of claims | Claim	19,286
2015 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 539	$ 480
IBNR	$ 278
Cumulative number of claims | Claim	16,798
2016 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Total	$ 469
IBNR	$ 400
Cumulative number of claims | Claim	11,600