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Reserve for Claims and Claim Expenses (Schedule of Incurred and Paid Claims Development) (Details) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011
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Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance | $ 17,106,243 |  |  |  |  |  |  |  |  |
IBNR and ACR | 5,240,331 |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance | 9,996,625 |  |  |  |  |  |  |  |  |
Outstanding liabilities from accident year 2010 and prior, net of reinsurance | 365,294 |  |  |  |  |  |  |  |  |
Claims and claim expenses, net of reinsurance, from the Company's former Bermuda-based insurance operations | 508 |  |  |  |  |  |  |  |  |
Adjustment for unallocated loss adjustment expenses | 57,538 |  |  |  |  |  |  |  |  |
Unamortized fair value adjustments recorded in connection with acquisitions | (77,830) |  |  |  |  |  |  |  |  |
Liability for claims and claim expenses, net of reinsurance | 7,455,128 |  |  |  |  |  |  |  |  |
Property |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance | 7,881,758 |  |  |  |  |  |  |  |  |
IBNR and ACR | 2,188,508 |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance | 5,060,973 |  |  |  |  |  |  |  |  |
Outstanding liabilities from accident year 2010 and prior, net of reinsurance | 113,550 |  |  |  |  |  |  |  |  |
Adjustment for unallocated loss adjustment expenses | 13,717 |  |  |  |  |  |  |  |  |
Unamortized fair value adjustments recorded in connection with acquisitions | (11,795) |  |  |  |  |  |  |  |  |
Liability for claims and claim expenses, net of reinsurance | 2,936,257 |  |  |  |  |  |  |  |  |
Casualty and Specialty |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance | 9,224,485 |  |  |  |  |  |  |  |  |
IBNR and ACR | 3,051,823 |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance | 4,935,652 |  |  |  |  |  |  |  |  |
Outstanding liabilities from accident year 2010 and prior, net of reinsurance | 251,744 |  |  |  |  |  |  |  |  |
Adjustment for unallocated loss adjustment expenses | 43,821 |  |  |  |  |  |  |  |  |
Unamortized fair value adjustments recorded in connection with acquisitions | (66,035) |  |  |  |  |  |  |  |  |
Liability for claims and claim expenses, net of reinsurance | 4,518,363 |  |  |  |  |  |  |  |  |
Accident Year 2011 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance | 1,739,044 | $ 1,741,868 | $ 1,753,974 | $ 1,760,870 | $ 1,775,879 | $ 1,814,478 | $ 1,845,827 | $ 1,905,502 | $ 2,003,446 | $ 2,057,531
IBNR and ACR | 42,682 |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance | 1,654,513 | 1,642,690 | 1,626,868 | 1,606,091 | 1,566,774 | 1,521,666 | 1,398,853 | 1,204,812 | 766,658 | 370,083
Accident Year 2011 | Property |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance | 1,380,673 | 1,379,645 | 1,398,038 | 1,412,109 | 1,415,459 | 1,446,398 | 1,470,521 | 1,501,089 | 1,571,670 | 1,628,699
IBNR and ACR | 23,635 |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance | 1,346,335 | 1,339,798 | 1,332,319 | 1,326,439 | 1,303,092 | 1,278,942 | 1,188,978 | 1,025,947 | 627,932 | 305,318
Accident Year 2011 | Casualty and Specialty |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance | 358,371 | 362,223 | 355,936 | 348,761 | 360,420 | 368,080 | 375,306 | 404,413 | 431,776 | 428,832
IBNR and ACR | 19,047 |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance | 308,178 | 302,892 | 294,549 | 279,652 | 263,682 | 242,724 | 209,875 | 178,865 | 138,726 | $ 64,765
Accident Year 2012 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance | 900,001 | 916,065 | 910,998 | 904,408 | 902,432 | 930,761 | 962,397 | 1,026,595 | 1,142,390 |
IBNR and ACR | 33,600 |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance | 798,486 | 789,281 | 758,265 | 726,192 | 651,403 | 599,696 | 525,208 | 419,242 | 268,952 |
Accident Year 2012 | Property |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance | 327,407 | 335,699 | 340,218 | 348,108 | 359,890 | 376,811 | 397,075 | 431,482 | 562,094 |
IBNR and ACR | 3,286 |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance | 317,291 | 316,691 | 311,179 | 307,845 | 293,008 | 281,594 | 254,924 | 206,440 | 166,143 |
Accident Year 2012 | Casualty and Specialty |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance | 572,594 | 580,366 | 570,780 | 556,300 | 542,542 | 553,950 | 565,322 | 595,113 | 580,296 |
IBNR and ACR | 30,314 |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance | 481,195 | 472,590 | 447,086 | 418,347 | 358,395 | 318,102 | 270,284 | 212,802 | $ 102,809 |
Accident Year 2013 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance | 736,657 | 738,973 | 755,412 | 774,095 | 798,648 | 845,473 | 896,904 | 921,455 |  |
IBNR and ACR | 22,627 |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance | 645,255 | 625,156 | 596,950 | 562,764 | 503,603 | 440,266 | 345,709 | 133,356 |  |
Accident Year 2013 | Property |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance | 246,855 | 240,252 | 240,007 | 243,839 | 255,322 | 277,641 | 300,349 | 323,743 |  |
IBNR and ACR | 1,177 |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance | 232,882 | 227,412 | 223,822 | 220,333 | 212,431 | 196,171 | 158,689 | 81,344 |  |
Accident Year 2013 | Casualty and Specialty |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance | 489,802 | 498,721 | 515,405 | 530,256 | 543,326 | 567,832 | 596,555 | 597,712 |  |
IBNR and ACR | 21,450 |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance | 412,373 | 397,744 | 373,128 | 342,431 | 291,172 | 244,095 | 187,020 | $ 52,012 |  |
Accident Year 2014 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance | 892,597 | 926,802 | 929,248 | 948,224 | 972,429 | 982,016 | 1,010,256 |  |  |
IBNR and ACR | 62,812 |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance | 777,240 | 747,069 | 699,620 | 637,889 | 561,274 | 438,641 | 231,666 |  |  |
Accident Year 2014 | Property |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance | 250,617 | 258,791 | 261,522 | 262,696 | 267,795 | 281,496 | 305,033 |  |  |
IBNR and ACR | 1,442 |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance | 247,165 | 247,578 | 243,862 | 236,789 | 224,960 | 185,934 | 107,075 |  |  |
Accident Year 2014 | Casualty and Specialty |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance | 641,980 | 668,011 | 667,726 | 685,528 | 704,634 | 700,520 | 705,223 |  |  |
IBNR and ACR | 61,370 |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance | 530,075 | 499,491 | 455,758 | 401,100 | 336,314 | 252,707 | $ 124,591 |  |  |
Accident Year 2015 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance | 1,145,196 | 1,132,034 | 1,136,753 | 1,167,569 | 1,151,413 | 1,146,737 |  |  |  |
IBNR and ACR | 111,924 |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance | 948,787 | 877,359 | 780,987 | 664,213 | 499,116 | 263,046 |  |  |  |
Accident Year 2015 | Property |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance | 310,317 | 314,389 | 325,719 | 336,705 | 359,662 | 375,399 |  |  |  |
IBNR and ACR | 5,495 |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance | 297,004 | 293,140 | 282,362 | 262,702 | 228,285 | 127,314 |  |  |  |
Accident Year 2015 | Casualty and Specialty |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance | 834,879 | 817,645 | 811,034 | 830,864 | 791,751 | 771,338 |  |  |  |
IBNR and ACR | 106,429 |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance | 651,783 | 584,219 | 498,625 | 401,511 | 270,831 | $ 135,732 |  |  |  |
Accident Year 2016 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance | 1,403,513 | 1,454,217 | 1,457,825 | 1,475,540 | 1,429,056 |  |  |  |  |
IBNR and ACR | 136,821 |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance | 1,089,800 | 978,676 | 837,022 | 629,894 | 288,221 |  |  |  |  |
Accident Year 2016 | Property |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance | 419,308 | 438,040 | 457,052 | 473,708 | 460,422 |  |  |  |  |
IBNR and ACR | 18,718 |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance | 374,565 | 354,088 | 329,543 | 260,902 | 120,478 |  |  |  |  |
Accident Year 2016 | Casualty and Specialty |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance | 984,205 | 1,016,177 | 1,000,773 | 1,001,832 | 968,634 |  |  |  |  |
IBNR and ACR | 118,103 |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance | 715,235 | 624,588 | 507,479 | 368,992 | $ 167,743 |  |  |  |  |
Accident Year 2017 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance | 2,685,050 | 2,691,020 | 2,761,844 | 2,966,271 |  |  |  |  |  |
IBNR and ACR | 543,257 |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance | 1,732,228 | 1,377,696 | 1,077,845 | 748,714 |  |  |  |  |  |
Accident Year 2017 | Property |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance | 1,364,285 | 1,360,897 | 1,467,835 | 1,649,071 |  |  |  |  |  |
IBNR and ACR | 276,858 |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance | 958,207 | 823,683 | 665,330 | 534,752 |  |  |  |  |  |
Accident Year 2017 | Casualty and Specialty |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance | 1,320,765 | 1,330,123 | 1,294,009 | 1,317,200 |  |  |  |  |  |
IBNR and ACR | 266,399 |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance | 774,021 | 554,013 | 412,515 | $ 213,962 |  |  |  |  |  |
Accident Year 2018 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance | 2,394,667 | 2,380,979 | 2,225,968 |  |  |  |  |  |  |
IBNR and ACR | 656,507 |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance | 1,257,609 | 819,173 | 589,357 |  |  |  |  |  |  |
Accident Year 2018 | Property |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance | 1,051,617 | 1,062,871 | 957,622 |  |  |  |  |  |  |
IBNR and ACR | 183,781 |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance | 667,336 | 454,553 | 432,075 |  |  |  |  |  |  |
Accident Year 2018 | Casualty and Specialty |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance | 1,343,050 | 1,318,108 | 1,268,346 |  |  |  |  |  |  |
IBNR and ACR | 472,726 |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance | 590,273 | 364,620 | $ 157,282 |  |  |  |  |  |  |
Accident Year 2019 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance | 2,229,128 | 2,234,727 |  |  |  |  |  |  |  |
IBNR and ACR | 1,167,735 |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance | 701,982 | 288,435 |  |  |  |  |  |  |  |
Accident Year 2019 | Property |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance | 978,600 | 1,014,100 |  |  |  |  |  |  |  |
IBNR and ACR | 427,179 |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance | 377,564 | 160,915 |  |  |  |  |  |  |  |
Accident Year 2019 | Casualty and Specialty |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance | 1,250,528 | 1,220,627 |  |  |  |  |  |  |  |
IBNR and ACR | 740,556 |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance | 324,418 | $ 127,520 |  |  |  |  |  |  |  |
Accident Year 2020 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance | 2,980,390 |  |  |  |  |  |  |  |  |
IBNR and ACR | 2,462,366 |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance | 390,725 |  |  |  |  |  |  |  |  |
Accident Year 2020 | Property |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance | 1,552,079 |  |  |  |  |  |  |  |  |
IBNR and ACR | 1,246,937 |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance | 242,624 |  |  |  |  |  |  |  |  |
Accident Year 2020 | Casualty and Specialty |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance | 1,428,311 |  |  |  |  |  |  |  |  |
IBNR and ACR | 1,215,429 |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance | $ 148,101 |  |  |  |  |  |  |  |  |

X
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- Definition Undiscounted amount, after reinsurance, of incurred-but-not-reported (IBNR) liabilities plus expected development on reported claims, for claims and allocated claim adjustment expense for short-duration insurance contracts. Excludes unallocated claim adjustment expense. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4E -URI http://asc.fasb.org/extlink&oid=116884468&loc=SL65671321-158438 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4D -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=116884468&loc=SL65671317-158438 + Details
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- Definition Undiscounted amount, after reinsurance, of the liability for unpaid claims and allocated claim adjustment expense for short-duration insurance contracts for accident years not separately presented in claim development information. Excludes unallocated claim adjustment expense. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4B -URI http://asc.fasb.org/extlink&oid=116884468&loc=SL65671311-158438 + Details
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