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Reserve for Claims and Claim Expenses (Schedule of Incurred and Paid Claims Development) (Details) - USD ($) $ in Thousands | Dec. 31, 2021 | 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 |  | $ 19,054,274 |  |  |  |  |  |  |  |  |
IBNR and ACR | $ 6,438,015 |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance |  | 10,353,691 |  |  |  |  |  |  |  |  |
Outstanding liabilities from accident year 2011 and prior, net of reinsurance |  | 341,797 |  |  |  |  |  |  |  |  |
Adjustment for unallocated loss adjustment expenses |  | 61,251 |  |  |  |  |  |  |  |  |
Unamortized fair value adjustments recorded in connection with acquisitions |  | (77,670) |  |  |  |  |  |  |  |  |
Liability for claims and claim expenses, net of reinsurance |  | 9,025,961 |  |  |  |  |  |  |  |  |
Property |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance |  | 8,518,921 |  |  |  |  |  |  |  |  |
IBNR and ACR | 2,734,172 |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance |  | 4,783,891 |  |  |  |  |  |  |  |  |
Outstanding liabilities from accident year 2011 and prior, net of reinsurance |  | 89,538 |  |  |  |  |  |  |  |  |
Adjustment for unallocated loss adjustment expenses |  | 16,266 |  |  |  |  |  |  |  |  |
Unamortized fair value adjustments recorded in connection with acquisitions |  | (11,642) |  |  |  |  |  |  |  |  |
Liability for claims and claim expenses, net of reinsurance |  | 3,829,192 |  |  |  |  |  |  |  |  |
Casualty and Specialty |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance |  | 10,535,353 |  |  |  |  |  |  |  |  |
IBNR and ACR | 3,703,843 |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance |  | 5,569,800 |  |  |  |  |  |  |  |  |
Outstanding liabilities from accident year 2011 and prior, net of reinsurance |  | 252,259 |  |  |  |  |  |  |  |  |
Adjustment for unallocated loss adjustment expenses |  | 44,985 |  |  |  |  |  |  |  |  |
Unamortized fair value adjustments recorded in connection with acquisitions |  | (66,028) |  |  |  |  |  |  |  |  |
Liability for claims and claim expenses, net of reinsurance |  | 5,196,769 |  |  |  |  |  |  |  |  |
Accident Year 2012 |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance |  | 893,612 | $ 894,228 | $ 911,821 | $ 907,765 | $ 901,147 | $ 899,241 | $ 927,397 | $ 958,667 | $ 1,022,338 | $ 1,138,478
IBNR and ACR | 28,520 |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance |  | 808,734 | 792,519 | 785,271 | 754,527 | 723,431 | 648,754 | 597,103 | 522,714 | 417,140 | 267,808
Accident Year 2012 | Property |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance |  | 322,871 | 325,042 | 334,347 | 338,877 | 346,756 | 358,509 | 375,439 | 395,605 | 429,885 | 560,348
IBNR and ACR | 1,180 |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance |  | 318,295 | 314,720 | 314,418 | 309,133 | 306,718 | 291,901 | 280,519 | 254,000 | 205,814 | 165,951
Accident Year 2012 | Casualty and Specialty |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance |  | 570,741 | 569,186 | 577,474 | 568,888 | 554,391 | 540,732 | 551,958 | 563,062 | 592,453 | 578,130
IBNR and ACR | 27,340 |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance |  | 490,439 | 477,799 | 470,853 | 445,394 | 416,713 | 356,853 | 316,584 | 268,714 | 211,326 | $ 101,857
Accident Year 2013 |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance |  | 726,277 | 720,503 | 726,212 | 747,939 | 766,453 | 790,498 | 836,813 | 887,176 | 912,458 |
IBNR and ACR | 16,529 |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance |  | 648,716 | 633,689 | 617,109 | 589,784 | 556,485 | 497,296 | 434,633 | 341,769 | 132,207 |
Accident Year 2013 | Property |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance |  | 240,779 | 238,404 | 235,356 | 235,016 | 238,734 | 250,014 | 272,191 | 294,315 | 318,033 |
IBNR and ACR | 1,113 |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance |  | 226,353 | 226,035 | 221,255 | 218,446 | 215,758 | 207,963 | 192,225 | 156,204 | 80,397 |
Accident Year 2013 | Casualty and Specialty |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance |  | 485,498 | 482,099 | 490,856 | 512,923 | 527,719 | 540,484 | 564,622 | 592,861 | 594,425 |
IBNR and ACR | 15,416 |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance |  | 422,363 | 407,654 | 395,854 | 371,338 | 340,727 | 289,333 | 242,408 | 185,565 | $ 51,810 |
Accident Year 2014 |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance |  | 888,622 | 897,645 | 932,269 | 922,659 | 941,733 | 965,706 | 974,640 | 1,002,755 |  |
IBNR and ACR | 57,970 |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance |  | 788,590 | 767,927 | 741,271 | 694,155 | 633,294 | 557,091 | 435,331 | 231,084 |  |
Accident Year 2014 | Property |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance |  | 247,708 | 250,647 | 256,845 | 259,379 | 260,542 | 265,569 | 278,813 | 302,158 |  |
IBNR and ACR | 372 |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance |  | 246,690 | 243,885 | 244,721 | 241,266 | 234,933 | 223,197 | 184,711 | 106,781 |  |
Accident Year 2014 | Casualty and Specialty |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance |  | 640,914 | 646,998 | 675,424 | 663,280 | 681,191 | 700,137 | 695,827 | 700,597 |  |
IBNR and ACR | 57,598 |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance |  | 541,900 | 524,042 | 496,550 | 452,889 | 398,361 | 333,894 | 250,620 | $ 124,303 |  |
Accident Year 2015 |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance |  | 1,111,987 | 1,117,250 | 1,105,473 | 1,130,597 | 1,161,202 | 1,144,947 | 1,139,588 |  |  |
IBNR and ACR | 73,388 |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance |  | 982,966 | 934,337 | 870,708 | 775,007 | 660,456 | 496,234 | 262,301 |  |  |
Accident Year 2015 | Property |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance |  | 295,081 | 305,847 | 311,964 | 323,211 | 334,099 | 357,065 | 372,338 |  |  |
IBNR and ACR | 71 |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance |  | 294,651 | 292,033 | 288,678 | 278,452 | 260,858 | 226,722 | 126,972 |  |  |
Accident Year 2015 | Casualty and Specialty |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance |  | 816,906 | 811,403 | 793,509 | 807,386 | 827,103 | 787,882 | 767,250 |  |  |
IBNR and ACR | 73,317 |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance |  | 688,315 | 642,304 | 582,030 | 496,555 | 399,598 | 269,512 | $ 135,329 |  |  |
Accident Year 2016 |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance |  | 1,373,871 | 1,366,532 | 1,420,715 | 1,447,703 | 1,464,953 | 1,418,381 |  |  |  |
IBNR and ACR | 59,360 |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance |  | 1,184,679 | 1,087,441 | 973,074 | 831,642 | 625,995 | 287,753 |  |  |  |
Accident Year 2016 | Property |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance |  | 411,698 | 415,572 | 434,706 | 452,922 | 469,120 | 455,503 |  |  |  |
IBNR and ACR | 16,889 |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance |  | 384,932 | 374,160 | 351,523 | 327,034 | 259,131 | 120,506 |  |  |  |
Accident Year 2016 | Casualty and Specialty |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance |  | 962,173 | 950,960 | 986,009 | 994,781 | 995,833 | 962,878 |  |  |  |
IBNR and ACR | 42,471 |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance |  | 799,747 | 713,281 | 621,551 | 504,608 | 366,864 | $ 167,247 |  |  |  |
Accident Year 2017 |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance |  | 2,559,381 | 2,603,328 | 2,664,387 | 2,748,704 | 2,954,415 |  |  |  |  |
IBNR and ACR | 342,654 |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance |  | 1,905,621 | 1,735,535 | 1,370,642 | 1,073,216 | 747,272 |  |  |  |  |
Accident Year 2017 | Property |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance |  | 1,273,461 | 1,328,419 | 1,350,684 | 1,461,953 | 1,644,982 |  |  |  |  |
IBNR and ACR | 217,210 |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance |  | 972,239 | 946,934 | 820,148 | 663,381 | 534,631 |  |  |  |  |
Accident Year 2017 | Casualty and Specialty |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance |  | 1,285,920 | 1,274,909 | 1,313,703 | 1,286,751 | 1,309,433 |  |  |  |  |
IBNR and ACR | 125,444 |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance |  | 933,382 | 788,601 | 550,494 | 409,835 | $ 212,641 |  |  |  |  |
Accident Year 2018 |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance |  | 2,188,749 | 2,297,920 | 2,342,952 | 2,198,790 |  |  |  |  |  |
IBNR and ACR | 371,723 |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance |  | 1,452,144 | 1,172,692 | 800,773 | 590,671 |  |  |  |  |  |
Accident Year 2018 | Property |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance |  | 857,217 | 979,598 | 1,020,102 | 938,309 |  |  |  |  |  |
IBNR and ACR | 105,483 |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance |  | 638,149 | 611,268 | 439,783 | 434,358 |  |  |  |  |  |
Accident Year 2018 | Casualty and Specialty |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance |  | 1,331,532 | 1,318,322 | 1,322,850 | 1,260,481 |  |  |  |  |  |
IBNR and ACR | 266,240 |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance |  | 813,995 | 561,424 | 360,990 | $ 156,313 |  |  |  |  |  |
Accident Year 2019 |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance |  | 2,154,462 | 2,213,257 | 2,255,467 |  |  |  |  |  |  |
IBNR and ACR | 834,344 |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance |  | 986,835 | 692,429 | 285,660 |  |  |  |  |  |  |
Accident Year 2019 | Property |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance |  | 898,472 | 956,445 | 992,526 |  |  |  |  |  |  |
IBNR and ACR | 254,099 |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance |  | 526,314 | 364,761 | 160,141 |  |  |  |  |  |  |
Accident Year 2019 | Casualty and Specialty |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance |  | 1,255,990 | 1,256,812 | 1,262,941 |  |  |  |  |  |  |
IBNR and ACR | 580,245 |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance |  | 460,521 | 327,668 | $ 125,519 |  |  |  |  |  |  |
Accident Year 2020 |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance |  | 3,076,722 | 3,090,954 |  |  |  |  |  |  |  |
IBNR and ACR | 1,562,397 |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance |  | 1,023,330 | 410,482 |  |  |  |  |  |  |  |
Accident Year 2020 | Property |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance |  | 1,600,743 | 1,580,564 |  |  |  |  |  |  |  |
IBNR and ACR | 585,134 |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance |  | 676,472 | 255,268 |  |  |  |  |  |  |  |
Accident Year 2020 | Casualty and Specialty |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance |  | 1,475,979 | 1,510,390 |  |  |  |  |  |  |  |
IBNR and ACR | 977,263 |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance |  | 346,858 | $ 155,214 |  |  |  |  |  |  |  |
Accident Year 2021 |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance |  | 4,080,591 |  |  |  |  |  |  |  |  |
IBNR and ACR | 3,091,130 |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance |  | 572,076 |  |  |  |  |  |  |  |  |
Accident Year 2021 | Property |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance |  | 2,370,891 |  |  |  |  |  |  |  |  |
IBNR and ACR | 1,552,621 |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance |  | 499,796 |  |  |  |  |  |  |  |  |
Accident Year 2021 | Casualty and Specialty |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |
Incurred Claims and Claim Expenses, Net of Reinsurance |  | 1,709,700 |  |  |  |  |  |  |  |  |
IBNR and ACR | $ 1,538,509 |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Claim Expenses, Net of Reinsurance |  | $ 72,280 |  |  |  |  |  |  |  |  |

X
- Definition Short-duration Insurance Contracts, Liability for Unpaid Claims and Claim Adjustment Expense, Unamortized Fair Value Adjustments Associated With Acquisition + References No definition available. + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition Amount, after reinsurance, of paid claims and allocated claim adjustment expense used in claims development 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 4B -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=116884468&loc=SL65671311-158438 + Details
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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 incurred claims and allocated claim adjustment expense used in claims development 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 4B -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=116884468&loc=SL65671311-158438 + Details
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