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Unpaid losses and loss expenses (A&E Loss Roll-forward) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Liability For Asbestos And Environmental Claims Net Roll Forward |  |  |  |
Balance (gross) at beginning of year |  | $ 1,988 | $ 2,117 | $ 2,228
Balance (net) at beginning of year |  | 1,326 | 1,447 | 1,527
Incurred activity, gross |  | 229 | 175 | 237
Incurred activity, net | [1] | 131 | 98 | (22)
Paid activity, gross |  | (349) | (304) | (348)
Paid activity, net |  | (205) | (219) | (58)
Balance (gross) at end of year |  | 1,868 | 1,988 | 2,117
Balance (net) at end of year |  | 1,252 | 1,326 | 1,447
Asbestos Issue [Member] |  |  |  |
Liability For Asbestos And Environmental Claims Net Roll Forward |  |  |  |
Balance (gross) at beginning of year |  | 1,459 | 1,492 | 1,621
Balance (net) at beginning of year |  | 916 | 964 | 1,051
Incurred activity, gross |  | 150 | 129 | 136
Incurred activity, net |  | 90 | 70 | 75
Paid activity, gross |  | (258) | (162) | (265)
Paid activity, net |  | (133) | (118) | (162)
Balance (gross) at end of year |  | 1,351 | 1,459 | 1,492
Balance (net) at end of year |  | 873 | 916 | 964
Environmental Issue [Member] |  |  |  |
Liability For Asbestos And Environmental Claims Net Roll Forward |  |  |  |
Balance (gross) at beginning of year |  | 529 | 625 | 607
Balance (net) at beginning of year |  | 410 | 483 | 476
Incurred activity, gross |  | 79 | 46 | 101
Incurred activity, net |  | 41 | 28 | (97)
Paid activity, gross |  | (91) | (142) | (83)
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease) |  | (72) | (101) | 104
Balance (gross) at end of year |  | 517 | 529 | 625
Balance (net) at end of year |  | 379 | 410 | $ 483
Brandywine [Member] |  |  |  |
Liability For Asbestos And Environmental Claims Net Roll Forward |  |  |  |
Balance (net) at beginning of year |  | 754 |  |
Balance (net) at end of year |  | 736 | 754 |
Westchester Specialty [Member] |  |  |  |
Liability For Asbestos And Environmental Claims Net Roll Forward |  |  |  |
Balance (net) at beginning of year |  | 117 |  |
Balance (net) at end of year |  | 103 | 117 |
Other Segments [Member] |  |  |  |
Liability For Asbestos And Environmental Claims Net Roll Forward |  |  |  |
Balance (net) at beginning of year |  | 74 |  |
Balance (net) at end of year |  | 80 | 74 |
The Chubb Corporation [Member] |  |  |  |
Liability For Asbestos And Environmental Claims Net Roll Forward |  |  |  |
Balance (net) at beginning of year |  | 381 |  |
Balance (net) at end of year |  | $ 333 | $ 381 |
[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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (cc) -URI http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437 + Details
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