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Reserve for Losses and Loss Adjustment Expenses - Reconciliation of beginning and ending balances of losses and loss adjustment reserves (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2020 | Sep. 30, 2019
Liability for Future Policy Benefits and Unpaid Claims and Claims Adjustment Expense [Abstract] |  |  |  |  |
Reserve for losses and loss adjustment expenses at beginning of period |  | $ 15,044,874 | $ 12,230,316 | $ 13,891,842 | $ 11,853,297
Unpaid losses and loss adjustment expenses recoverable |  | 4,156,157 | 3,024,797 | 4,082,650 | 2,814,291
Net reserve for losses and loss adjustment expenses at beginning of period |  | 10,888,717 | 9,205,519 | 9,809,192 | 9,039,006
Net incurred losses and loss adjustment expenses relating to losses occurring in: |  |  |  |  |
Current year |  | 1,264,315 | 855,352 | 3,673,346 | 2,419,044
Prior years |  | (48,042) | (52,897) | (111,132) | (130,514)
Total net incurred losses and loss adjustment expenses |  | 1,216,273 | 802,455 | 3,562,214 | 2,288,530
Retroactive reinsurance transaction | [1] | 0 | 0 | 60,635 | (225,500)
Net foreign exchange (gains) losses |  | 114,122 | (73,200) | 22,706 | (74,981)
Net paid losses and loss adjustment expenses relating to losses occurring in: |  |  |  |  |
Current year |  | (189,961) | (175,611) | (359,395) | (301,099)
Prior years |  | (512,263) | (399,948) | (1,578,464) | (1,366,741)
Total net paid losses and loss adjustment expenses |  | (702,224) | (575,559) | (1,937,859) | (1,667,840)
Net reserve for losses and loss adjustment expenses at end of period |  | 11,516,888 | 9,359,215 | 11,516,888 | 9,359,215
Unpaid losses and loss adjustment expenses recoverable |  | 4,383,638 | 3,030,169 | 4,383,638 | 3,030,169
Reserve for losses and loss adjustment expenses at end of period |  | $ 15,900,526 | $ 12,389,384 | $ 15,900,526 | $ 12,389,384
[1] | During 2020 first quarter, a subsidiary of the Company entered into a reinsurance to close agreement of the 2017 and prior years of account previously covered by a third party arrangement, while in the 2019 first quarter, a subsidiary of the Company entered into a retroactive reinsurance transaction with third party reinsurer to reinsure run-off liabilities associated with certain U.S. insurance exposures.

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- Definition Amount of expense (reversal of expense) for claims incurred and related claims settlement costs for retroactive reinsurance and discount accretion. + References No definition available. + Details
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- Definition Represents the sum of a) The known and estimated amount recoverable as of the balance sheet date from reinsurers for claims paid or incurred by the ceding insurer and associated claims settlement expenses, including estimated amounts for claims incurred but not reported, and policy benefits, net of any related valuation allowance, and b) the excess of the estimated liabilities for claims and claim costs over the consideration received for retroactive reinsurance contracts as of the balance sheet date. + References No definition available. + Details
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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 Amount, after effects of reinsurance, of payments to settle claims incurred in the current period and related claims settlement costs. + 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 (c) -URI http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437 + Details
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- Definition Amount, after effects of reinsurance, of payments to settle claims incurred in prior periods and related claims settlement costs. + 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 (c) -URI http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437 + Details
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- Definition Amount of foreign currency gain (loss) which decreases (increases) the cost of settling unpaid claims after deduction of reinsurance recoveries. Includes, but is not limited to, claims which have been incurred but not reported. + References No definition available. + 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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- Definition Amount, after effects of reinsurance, of expense for claims incurred in the current reporting period and related claims settlement costs. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437 + Details
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