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Unpaid losses and loss expenses (RF) (Details) - USD ($) $ in Millions | 3 Months Ended
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Mar. 31, 2021 | Mar. 31, 2020
Unpaid Losses and Loss Expenses [Roll Forward] |  |  |
Gross unpaid losses and loss expenses – beginning of period |  | $ 67,811 | $ 62,690
Reinsurance recoverable on unpaid losses – beginning of period (1) |  | (14,647) | (14,181)
Net unpaid losses and loss expenses – beginning of period |  | 53,164 | 48,509
Current Year Claims and Claims Adjustment Expense |  | 5,249 | 4,605
Prior Year Claims and Claims Adjustment Expense | [1] | (196) | (120)
Total, Incurred |  | 5,053 | 4,485
Net loss and loss expenses paid, Current Year |  | 732 | 920
Net loss and loss expenses paid, Prior Years |  | 3,161 | 3,335
Total, Paid |  | 3,893 | 4,255
Liability For Unpaid Claims And Claims Adjustment Expense Foreign Currency Revaluation And Other |  | 71 | (565)
Net unpaid losses and loss expenses – end of period |  | 54,395 | 48,174
Reinsurance recoverable on unpaid losses (1) | [2] | 14,860 | 14,040
Gross unpaid losses and loss expenses – end of period |  | 69,255 | 62,214
prior period development, net adjustments |  | 4 | $ 2
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease), Gross |  | 1,444 |
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease) |  | $ 1,231 |
[1] | Relates to prior period loss reserve development only and excludes prior period development related to reinstatement premiums, expense adjustments and earned premiums totaling $4 million and $2 million for the three months ended March 31, 2021 and 2020, respectively.
[2] | Net of valuation allowance for uncollectible reinsurance.

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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=120400017&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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