Unpaid losses and loss expenses (A&E Loss Roll-forward) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015
Liability For Asbestos And Environmental Claims Net Roll Forward
Balance (gross) at December 31, 2015	$ 1,550
Balance (net) at December 31, 2015	980
Acquired reserves, gross	859
Acquired reserves, net	822
Incurred activity, gross	415
Incurred activity, net	164	$ 162
Paid activity, gross	(521)
Paid activity, net	(357)
Balance (gross) at December 31, 2016	2,303	1,550
Balance (net) at December 31, 2016	1,609	980
Brandywine [Member]
Liability For Asbestos And Environmental Claims Net Roll Forward
Balance (net) at December 31, 2015	782
Balance (net) at December 31, 2016	760	782
Westchester Specialty [Member]
Liability For Asbestos And Environmental Claims Net Roll Forward
Balance (net) at December 31, 2015	115
Balance (net) at December 31, 2016	112	115
Chubb Corp [Member]
Liability For Asbestos And Environmental Claims Net Roll Forward
Balance (net) at December 31, 2015	0
Balance (net) at December 31, 2016	657	0
Other Segments [Member]
Liability For Asbestos And Environmental Claims Net Roll Forward
Balance (net) at December 31, 2015	83
Balance (net) at December 31, 2016	80	83
Environmental
Liability For Asbestos And Environmental Claims Net Roll Forward
Balance (gross) at December 31, 2015	199
Balance (net) at December 31, 2015	149
Acquired reserves, gross	371
Acquired reserves, net	354
Incurred activity, gross	104
Incurred activity, net	69
Paid activity, gross	(97)
Paid activity, net	(82)
Balance (gross) at December 31, 2016	577	199
Balance (net) at December 31, 2016	490	149
Asbestos
Liability For Asbestos And Environmental Claims Net Roll Forward
Balance (gross) at December 31, 2015	1,351
Balance (net) at December 31, 2015	831
Acquired reserves, gross	488
Acquired reserves, net	468
Incurred activity, gross	311
Incurred activity, net	95
Paid activity, gross	(424)
Paid activity, net	(275)
Balance (gross) at December 31, 2016	1,726	1,351
Balance (net) at December 31, 2016	$ 1,119	$ 831

Carrying amount as of the balance sheet date of reserves for the costs of settling insured claims and costs incurred in the claims settlement process attributable to asbestos and environmental claims, before estimated recoveries from reinsurers.

Reference 1: 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

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

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (cc)
-URI http://asc.fasb.org/extlink&oid=65670824&loc=d3e14764-158437

Reference 2: http://www.xbrl.org/2003/role/presentationRef
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-Section 50
-Paragraph 3
-Subparagraph (a)
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