Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2017	Sep. 30, 2017	Dec. 31, 2016
Commitments [Line Items]
Future policy benefits	$ 11,040	$ 11,040	$ 10,326
CNA Financial [Member]
Commitments [Line Items]
Aggregate amount related to indemnification agreements	254	254
Aggregate amount related to quantifiable guarantees	375	375
Potential amount of future payments under guarantees	1,800	1,800
Increase (decrease) in premium development recognized	0	(37)
Decrease in pretax income	1	7
CNA Financial [Member] \| Workers Compensation Policy [Member]
Commitments [Line Items]
Future policy benefits	$ 61	$ 61

Amount of accrued obligation to policyholders that relates to insured events for long-duration contracts.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 25
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=28366191&loc=d3e13145-158430

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (cc)
-URI http://asc.fasb.org/extlink&oid=65670824&loc=d3e14764-158437

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.13(a)(1))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=65670824&loc=d3e14764-158437

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