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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 | 6 Months Ended | 9 Months Ended
--- | --- | --- | ---
Sep. 30, 2021 | Sep. 30, 2020 | Jun. 30, 2021 | Sep. 30, 2021 | Sep. 30, 2020
Liability for Future Policy Benefits and Unpaid Claims and Claims Adjustment Expense [Abstract] |  |  |  |  |  |
Reserve for losses and loss adjustment expenses at beginning of period |  | $ 17,196,648 | $ 15,044,874 | $ 16,513,929 | $ 16,513,929 | $ 13,891,842
Unpaid losses and loss adjustment expenses recoverable |  | 4,146,020 | 4,156,157 | 4,314,855 | 4,314,855 | 4,082,650
Net reserve for losses and loss adjustment expenses at beginning of period |  | 13,050,628 | 10,888,717 | 12,199,074 | 12,199,074 | 9,809,192
Net incurred losses and loss adjustment expenses relating to losses occurring in: |  |  |  |  |  |
Current year |  | 1,348,528 | 1,264,315 |  | 3,812,381 | 3,673,346
Prior years |  | (122,509) | (48,042) |  | (223,431) | (111,132)
Total net incurred losses and loss adjustment expenses |  | 1,226,019 | 1,216,273 |  | 3,588,950 | 3,562,214
Net losses and loss adjustment expense reserves of acquired business | [1] | 104,307 | 0 |  | 104,307 | 0
Retroactive reinsurance transaction | [2] | 0 | 0 |  | (183,893) | 60,635
Impact of deconsolidation of Watford |  | (1,460,611) | 0 |  | (1,460,611) | 0
Net foreign exchange (gains) losses |  | (78,152) | 114,122 |  | 10,818 | 22,706
Net paid losses and loss adjustment expenses relating to losses occurring in: |  |  |  |  |  |
Current year |  | (208,923) | (189,961) |  | (432,348) | (359,395)
Prior years |  | (417,368) | (512,263) |  | (1,610,397) | (1,578,464)
Total net paid losses and loss adjustment expenses |  | (626,291) | (702,224) |  | (2,042,745) | (1,937,859)
Net reserve for losses and loss adjustment expenses at end of period |  | 12,215,900 | 11,516,888 | 13,050,628 | 12,215,900 | 11,516,888
Unpaid losses and loss adjustment expenses recoverable |  | 5,115,147 | 4,383,638 | 4,146,020 | 5,115,147 | 4,383,638
Reserve for losses and loss adjustment expenses at end of period |  | $ 17,331,047 | $ 15,900,526 | $ 17,196,648 | $ 17,331,047 | $ 15,900,526
[1] | Represents activity related to the Company’s acquisitions in the 2021 period.
[2] | During the 2021 first quarter, the Company entered into a reinsurance to close and other related agreements with Premia Managing Agency Limited (“Premia”), in connection with the 2018 and prior years of account related to the acquisition of Barbican Group Holdings Limited (“Barbican”). During the 2020 first quarter, the Company entered into a reinsurance to close agreement of the 2017 and prior years of account previously covered by a third party arrangement.

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- Definition Reduction in reserves due to no longer including the former VIEs reserves in the consolidated entity's reserves. + References No definition available. + Details
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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 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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