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Unpaid losses and loss expenses (Unpaid Losses and Loss Expenses Rollforward) (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Unpaid Losses and Loss Expenses [Roll Forward] |  |  |  |
Gross unpaid losses and loss expenses, beginning of year |  | $ 62,960 | $ 63,179 | $ 60,540
Reinsurance recoverable on unpaid losses, beginning of year | [1] | (14,689) | (14,014) | (12,708)
Net unpaid losses and loss expenses, beginning of year |  | 48,271 | 49,165 | 47,832
Net losses and loss expenses incurred in respect of losses incurring in Current Year |  | 19,575 | 19,048 | 19,391
Prior Year Claims and Claims Adjustment Expense | [2] | (845) | (981) | (937)
Total |  | 18,730 | 18,067 | 18,454
Net losses and loss expenses paid in Current Year |  | 7,894 | 7,544 | 6,575
Net losses and loss expenses paid in Prior Year |  | 10,579 | 10,796 | 10,873
Total |  | 18,473 | 18,340 | 17,448
Foreign currency revaluation and other |  | (19) | (621) | 327
Net unpaid losses and loss expenses, end of year |  | 48,509 | 48,271 | 49,165
Reinsurance recoverable on unpaid losses, end of year | [1] | 14,181 | 14,689 | 14,014
Gross unpaid losses and loss expenses, end of year |  | 62,690 | 62,960 | 63,179
Prior Period Development, net adjustments |  | $ 53 | $ 85 | $ 108
[1] | Net of provision for uncollectible reinsurance.
[2] | Relates to prior period loss reserve development only and excludes prior period development related to reinstatement premiums, expense adjustments and earned premiums totaling $53 million, $85 million and $108 million for 2019, 2018, and 2017, respectively.

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- Definition The amount needed to reflect the estimated ultimate cost of settling claims relating to insured events that have occurred on or before the balance sheet date, whether or not reported to the insurer at that date. + 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 (a) -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 (cc) -URI http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437 + Details
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- Definition Liability as of the balance sheet date for amounts representing estimated cost of settling unpaid claims under the terms of the underlying insurance policies, less estimated reinsurance recoveries on such claims. This includes an estimate for claims which have been incurred but not reported. Claim adjustment expenses represent the costs estimated to be incurred in the settlement of unpaid claims. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(13)(a)) -URI http://asc.fasb.org/extlink&oid=119400593&loc=d3e572229-122910 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 (cc) -URI http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437 Reference 3: 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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