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Unpaid losses and loss expenses (A&E Loss Roll-forward) (Details) - USD ($) $ in Millions | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Liability for Catastrophe Claims [Line Items] |  |  |  |  |
Unpaid losses and loss expenses | $ 63,179 |  | $ 60,540 | $ 37,303 | $ 38,315
Liability For Asbestos And Environmental Claims Net Roll Forward |  |  |  |  |
Balance (gross) at December 31, 2016 | 2,303 |  |  |  |
Balance (net) at December 31, 2016 | 1,609 |  |  |  |
Incurred activity, gross | 427 |  |  |  |
Incurred activity, net | 217 | [1] | 164 |  |
Paid activity, gross | (502) |  |  |  |
Paid activity, net | (299) |  |  |  |
Balance (gross) at December 31, 2017 | 2,228 |  | 2,303 |  |
Balance (net) at December 31, 2017 | 1,527 |  | 1,609 |  |
Brandywine [Member] |  |  |  |  |
Liability For Asbestos And Environmental Claims Net Roll Forward |  |  |  |  |
Balance (net) at December 31, 2016 | 760 |  |  |  |
Balance (net) at December 31, 2017 | 849 |  | 760 |  |
Westchester Specialty [Member] |  |  |  |  |
Liability For Asbestos And Environmental Claims Net Roll Forward |  |  |  |  |
Balance (net) at December 31, 2016 | 112 |  |  |  |
Balance (net) at December 31, 2017 | 113 |  | 112 |  |
Chubb Corp [Member] |  |  |  |  |
Liability For Asbestos And Environmental Claims Net Roll Forward |  |  |  |  |
Balance (net) at December 31, 2016 | 657 |  |  |  |
Balance (net) at December 31, 2017 | 486 |  | 657 |  |
Other Segments [Member] |  |  |  |  |
Liability For Asbestos And Environmental Claims Net Roll Forward |  |  |  |  |
Balance (net) at December 31, 2016 | 80 |  |  |  |
Balance (net) at December 31, 2017 | 79 |  | 80 |  |
Environmental |  |  |  |  |
Liability For Asbestos And Environmental Claims Net Roll Forward |  |  |  |  |
Balance (gross) at December 31, 2016 | 577 |  |  |  |
Balance (net) at December 31, 2016 | 490 |  |  |  |
Incurred activity, gross | 199 |  |  |  |
Incurred activity, net | 113 |  |  |  |
Paid activity, gross | (169) |  |  |  |
Paid activity, net | (127) |  |  |  |
Balance (gross) at December 31, 2017 | 607 |  | 577 |  |
Balance (net) at December 31, 2017 | 476 |  | 490 |  |
Asbestos |  |  |  |  |
Liability For Asbestos And Environmental Claims Net Roll Forward |  |  |  |  |
Balance (gross) at December 31, 2016 | 1,726 |  |  |  |
Balance (net) at December 31, 2016 | 1,119 |  |  |  |
Incurred activity, gross | 228 |  |  |  |
Incurred activity, net | 104 |  |  |  |
Paid activity, gross | (333) |  |  |  |
Paid activity, net | (172) |  |  |  |
Balance (gross) at December 31, 2017 | 1,621 |  | 1,726 |  |
Balance (net) at December 31, 2017 | 1,051 |  | $ 1,119 |  |
Brandywine Run-off [Member] |  |  |  |  |
Liability for Catastrophe Claims [Line Items] |  |  |  |  |
Unpaid losses and loss expenses | $ 2,000 |  |  |  |
[1] | (1) Excludes unallocated loss expenses and the net activity reflects third-party reinsurance other than the aggregate excess of loss reinsurance provided by National Indemnity Company (NICO) to Westchester Specialty (see Westchester Specialty section below).

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- Definition 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. + References 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 + Details
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