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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 | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2020 | Sep. 30, 2020 | Jun. 30, 2020 | Mar. 31, 2020 | Dec. 31, 2019 | Sep. 30, 2019 | Jun. 30, 2019 | Mar. 31, 2019 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  | $ 1,811,045 | $ 1,771,092 | $ 1,665,354 | $ 1,744,444 | $ 1,515,882 | $ 1,438,023 | $ 1,463,727 | $ 1,368,866 | $ 6,991,935 | $ 5,786,498 | $ 5,231,975
Net premiums written |  | $ 1,758,015 | $ 1,874,144 | $ 1,668,311 | $ 2,137,246 | $ 1,455,453 | $ 1,613,457 | $ 1,444,898 | $ 1,525,259 | 7,437,716 | 6,039,067 | 5,346,747
Other |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  |  | 560,351 | 556,689 | 578,862
Net premiums written |  |  |  |  |  |  |  |  |  | 537,589 | 532,862 | 604,175
Other | United States |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net premiums written |  |  |  |  |  |  |  |  |  | 115,471 | 127,176 | 49,800
Other | Europe |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net premiums written |  |  |  |  |  |  |  |  |  | 97,753 | 52,065 | 91,635
Other | Bermuda |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net premiums written |  |  |  |  |  |  |  |  |  | 324,365 | 353,621 | 462,740
Other | Property excluding property catastrophe |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net premiums earned | [1] |  |  |  |  |  |  |  |  | 1,130 | 3,503 | 2,802
Other | Property catastrophe |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net premiums earned | [1] |  |  |  |  |  |  |  |  | 23,037 | 13,399 | 10,998
Other | Other specialty |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net premiums earned | [1],[2] |  |  |  |  |  |  |  |  | 186,717 | 185,547 | 204,485
Other | Casualty |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net premiums earned | [1],[3] |  |  |  |  |  |  |  |  | 245,272 | 246,894 | 277,589
Other | Marine and aviation |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  |  | 429 | 0 | 0
Other | Other |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net premiums earned | [1],[4] |  |  |  |  |  |  |  |  | 103,766 | 107,346 | 82,988
Underwriting segments |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  |  | 6,431,584 | 5,229,809 | 4,653,113
Net premiums written |  |  |  |  |  |  |  |  |  | 6,900,127 | 5,506,205 | 4,742,572
Underwriting segments | Insurance |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  |  | 2,871,420 | 2,397,080 | 2,205,661
Net premiums written |  |  |  |  |  |  |  |  |  | 3,162,907 | 2,641,726 | 2,212,125
Underwriting segments | Insurance | United States |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net premiums written | [5] |  |  |  |  |  |  |  |  | 2,158,415 | 1,983,476 | 1,736,651
Underwriting segments | Insurance | Europe |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net premiums written | [5] |  |  |  |  |  |  |  |  | 856,572 | 559,214 | 401,974
Underwriting segments | Insurance | Other geographic location |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net premiums written | [5] |  |  |  |  |  |  |  |  | 147,920 | 99,036 | 73,500
Underwriting segments | Insurance | Property, energy, marine and aviation |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net premiums earned | [5] |  |  |  |  |  |  |  |  | 517,247 | 298,966 | 205,069
Underwriting segments | Insurance | Professional lines |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net premiums earned | [5],[6] |  |  |  |  |  |  |  |  | 655,872 | 499,224 | 458,425
Underwriting segments | Insurance | Programs |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net premiums earned | [5] |  |  |  |  |  |  |  |  | 432,854 | 414,103 | 389,186
Underwriting segments | Insurance | Construction and national accounts |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net premiums earned | [5] |  |  |  |  |  |  |  |  | 387,934 | 325,687 | 322,440
Underwriting segments | Insurance | Excess and surplus casualty |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net premiums earned | [5],[7] |  |  |  |  |  |  |  |  | 270,620 | 200,615 | 172,424
Underwriting segments | Insurance | Travel, accident and health |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net premiums earned | [5] |  |  |  |  |  |  |  |  | 190,944 | 305,085 | 297,147
Underwriting segments | Insurance | Lenders products |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net premiums earned | [5] |  |  |  |  |  |  |  |  | 114,687 | 66,079 | 94,248
Underwriting segments | Insurance | Other |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net premiums earned | [5],[8] |  |  |  |  |  |  |  |  | 301,262 | 287,321 | 266,722
Underwriting segments | Reinsurance |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  |  | 2,162,229 | 1,466,389 | 1,261,216
Net premiums written |  |  |  |  |  |  |  |  |  | 2,457,370 | 1,602,723 | 1,372,572
Underwriting segments | Reinsurance | United States |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net premiums written | [9] |  |  |  |  |  |  |  |  | 687,622 | 529,943 | 413,550
Underwriting segments | Reinsurance | Bermuda |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net premiums written | [9] |  |  |  |  |  |  |  |  | 1,001,990 | 578,618 | 487,523
Underwriting segments | Reinsurance | Europe and other |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net premiums written |  |  |  |  |  |  |  |  |  | 767,758 | 494,162 | 471,499
Underwriting segments | Reinsurance | Property excluding property catastrophe |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net premiums earned | [9],[10] |  |  |  |  |  |  |  |  | 562,208 | 362,841 | 287,788
Underwriting segments | Reinsurance | Property catastrophe |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net premiums earned | [9],[11] |  |  |  |  |  |  |  |  | 237,736 | 90,934 | 75,249
Underwriting segments | Reinsurance | Other specialty |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net premiums earned | [9],[10] |  |  |  |  |  |  |  |  | 626,409 | 478,517 | 474,568
Underwriting segments | Reinsurance | Casualty |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net premiums earned | [9],[11] |  |  |  |  |  |  |  |  | 549,056 | 429,288 | 347,034
Underwriting segments | Reinsurance | Marine and aviation |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net premiums earned | [9] |  |  |  |  |  |  |  |  | 109,624 | 48,274 | 39,238
Underwriting segments | Reinsurance | Other |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net premiums earned | [9],[12] |  |  |  |  |  |  |  |  | 77,196 | 56,535 | 37,339
Underwriting segments | Mortgage |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  |  | 1,397,935 | 1,366,340 | 1,186,236
Net premiums written |  |  |  |  |  |  |  |  |  | 1,279,850 | 1,261,756 | 1,157,875
Underwriting segments | Mortgage | United States |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  |  | 1,158,563 | 1,134,849 | 1,009,765
Net premiums written |  |  |  |  |  |  |  |  |  | 1,021,950 | 1,032,868 | 948,323
Underwriting segments | Mortgage | Other geographic location |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  |  | 239,372 | 231,491 | 176,471
Net premiums written |  |  |  |  |  |  |  |  |  | $ 257,900 | $ 228,888 | $ 209,552
[1] | Other segment results include premiums assumed through intersegment transactions and exclude premiums ceded through intersegment transactions.
[2] | Includes proportional motor and other.
[3] | Includes professional liability, excess motor, programs and other.
[4] | Includes mortgage, US programs and other.
[5] | Insurance segment results include premiums assumed through intersegment transactions and exclude premiums ceded through intersegment transactions.
[6] | Includes professional liability, executive assurance and healthcare business.
[7] | Includes casualty and contract binding business.
[8] | Includes alternative markets, excess workers' compensation and surety business.
[9] | Reinsurance segment results include premiums assumed through intersegment transactions and exclude premiums ceded through intersegment transactions.
[10] | Includes proportional motor, surety, accident and health, workers’ compensation catastrophe, agriculture, trade credit and other.
[11] | Includes executive assurance, professional liability, workers’ compensation, excess motor, healthcare and other.
[12] | Includes life, casualty clash and other.

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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 http://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 http://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 http://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 http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 + Details
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