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Segment Information - Summary of net premiums written and earned by major line of business and net premiums written by location (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned |  | $ 9,678,077 | $ 8,082,298 | $ 6,991,935
Net premiums written |  | 11,077,189 | 9,018,337 | 7,437,716
Operating segments | Insurance |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned |  | 4,559,335 | 3,626,468 | 2,871,420
Net premiums written |  | 5,020,642 | 4,148,193 | 3,162,907
Operating segments | Insurance | United States |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums written | [1] | 3,340,038 | 2,813,039 | 2,158,415
Operating segments | Insurance | Europe |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums written | [1] | 1,405,719 | 1,125,192 | 856,572
Operating segments | Insurance | Other geographic location |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums written | [1] | 274,885 | 209,962 | 147,920
Operating segments | Insurance | Professional lines |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | [1] | 1,314,236 | 942,817 | 655,872
Operating segments | Insurance | Property, energy, marine and aviation |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | [1] | 772,388 | 667,892 | 506,182
Operating segments | Insurance | Programs |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | [1] | 589,860 | 506,867 | 432,854
Operating segments | Insurance | Travel, accident and health |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | [1] | 491,847 | 255,590 | 190,944
Operating segments | Insurance | Construction and national accounts |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | [1] | 432,020 | 416,107 | 398,999
Operating segments | Insurance | Excess and surplus casualty |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | [1] | 393,353 | 318,027 | 270,620
Operating segments | Insurance | Warranty and lenders solutions |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | [1] | 127,222 | 153,958 | 114,687
Operating segments | Insurance | Other |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | [1] | 438,409 | 365,210 | 301,262
Operating segments | Reinsurance |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned |  | 3,959,381 | 2,840,443 | 2,162,229
Net premiums written |  | 4,923,976 | 3,254,374 | 2,457,370
Operating segments | Reinsurance | United States |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums written | [1] | 1,246,507 | 828,504 | 687,622
Operating segments | Reinsurance | Bermuda |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums written | [1] | 2,561,771 | 1,557,294 | 1,001,990
Operating segments | Reinsurance | Europe and other |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums written | [1] | 1,115,698 | 868,576 | 767,758
Operating segments | Reinsurance | Other specialty |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | [1] | 1,377,880 | 818,801 | 626,409
Operating segments | Reinsurance | Property excluding property catastrophe |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | [1] | 1,091,440 | 836,573 | 562,208
Operating segments | Reinsurance | Casualty |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | [1] | 854,543 | 666,754 | 549,056
Operating segments | Reinsurance | Property catastrophe |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | [1] | 366,991 | 280,738 | 237,736
Operating segments | Reinsurance | Marine and aviation |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | [1] | 159,401 | 152,955 | 109,624
Operating segments | Reinsurance | Other |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | [1] | 109,126 | 84,622 | 77,196
Operating segments | Mortgage |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned |  | 1,159,361 | 1,283,419 | 1,397,935
Net premiums written |  | 1,132,571 | 1,261,068 | 1,279,850
Operating segments | Mortgage | United States |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned |  | 815,519 | 970,507 | 1,158,563
Net premiums written |  | 780,256 | 914,477 | 1,021,950
Operating segments | Mortgage | Other geographic location |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned |  | 343,842 | 312,912 | 239,372
Net premiums written |  | 352,315 | 346,591 | 257,900
Operating segments | Other |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned |  | 0 | 331,968 | 560,351
Net premiums written |  | 0 | 354,702 | 537,589
Operating segments | Other | United States |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums written | [1] | 0 | 63,403 | 115,471
Operating segments | Other | Europe |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums written | [1] | 0 | 91,499 | 97,753
Operating segments | Other | Bermuda |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums written | [1] | 0 | 199,800 | 324,365
Operating segments | Other | Other specialty |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | [1] | 0 | 118,356 | 186,717
Operating segments | Other | Property excluding property catastrophe |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | [1] | 0 | 6,578 | 1,130
Operating segments | Other | Casualty |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | [1] | 0 | 138,551 | 245,272
Operating segments | Other | Property catastrophe |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | [1] | 0 | 15,235 | 23,037
Operating segments | Other | Marine and aviation |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | [1] | 0 | 190 | 429
Operating segments | Other | Other |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | [1] | $ 0 | $ 53,058 | $ 103,766
[1] | Segment results include premiums assumed through intersegment transactions and exclude premiums ceded through intersegment transactions.

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- Definition Amount, after premiums ceded to other entities and premiums assumed by the entity, of premiums earned. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 220 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04(1)) -URI https://asc.fasb.org/extlink&oid=120400993&loc=SL114874131-224263 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 2 -Subparagraph (SX 210.12-17(Column E)) -URI https://asc.fasb.org/extlink&oid=120401096&loc=d3e574992-122915 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 220 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04(13)(c)) -URI https://asc.fasb.org/extlink&oid=120399700&loc=SL114874048-224260 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (c)(3) -URI https://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 + Details
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