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Reserve for Claims and Claim Expenses (Incurred and Paid Claims Development) (Details) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2009 | Dec. 31, 2008
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and claim expenses, net of reinsurance | $ 8,951,571 |  |  |  |  |  |  |  |  |
IBNR and ACR | 2,430,824 |  |  |  |  |  |  |  |  |
Cumulative paid claims and claim expenses, net of reinsurance | 5,772,376 |  |  |  |  |  |  |  |  |
Outstanding liabilities from accident year 2007 and prior, net of reinsurance | 282,782 |  |  |  |  |  |  |  |  |
Claims and claim expenses, net of reinsurance, from the Company's former Bermuda-based insurance operations | 673 |  |  |  |  |  |  |  |  |
Adjustment for unallocated claim expenses | 23,694 |  |  |  |  |  |  |  |  |
Unamortized fair value adjustments recorded in connection with the acquisition of Platinum | 7,434 |  |  |  |  |  |  |  |  |
Liability for claims and claim expenses, net of reinsurance | 3,493,778 |  |  |  |  |  |  |  |  |
Property |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and claim expenses, net of reinsurance | 4,809,181 |  |  |  |  |  |  |  |  |
IBNR and ACR | 1,104,634 |  |  |  |  |  |  |  |  |
Cumulative paid claims and claim expenses, net of reinsurance | 3,474,410 |  |  |  |  |  |  |  |  |
Outstanding liabilities from accident year 2007 and prior, net of reinsurance | 4,211 |  |  |  |  |  |  |  |  |
Adjustment for unallocated claim expenses | 3,352 |  |  |  |  |  |  |  |  |
Unamortized fair value adjustments recorded in connection with the acquisition of Platinum | 956 |  |  |  |  |  |  |  |  |
Liability for claims and claim expenses, net of reinsurance | 1,343,290 |  |  |  |  |  |  |  |  |
Casualty and Specialty |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and claim expenses, net of reinsurance | 4,142,390 |  |  |  |  |  |  |  |  |
IBNR and ACR | 1,326,190 |  |  |  |  |  |  |  |  |
Cumulative paid claims and claim expenses, net of reinsurance | 2,297,966 |  |  |  |  |  |  |  |  |
Outstanding liabilities from accident year 2007 and prior, net of reinsurance | 278,571 |  |  |  |  |  |  |  |  |
Adjustment for unallocated claim expenses | 20,342 |  |  |  |  |  |  |  |  |
Unamortized fair value adjustments recorded in connection with the acquisition of Platinum | 6,478 |  |  |  |  |  |  |  |  |
Liability for claims and claim expenses, net of reinsurance | 2,149,815 |  |  |  |  |  |  |  |  |
Accident Year 2008 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and claim expenses, net of reinsurance | 1,241,116 | $ 1,239,405 | $ 1,256,183 | $ 1,266,331 | $ 1,288,248 | $ 1,310,946 | $ 1,383,378 | $ 1,401,967 | $ 1,421,816 | $ 1,459,436
IBNR and ACR | 44,189 |  |  |  |  |  |  |  |  |
Cumulative paid claims and claim expenses, net of reinsurance | 1,164,647 | 1,145,149 | 1,123,287 | 1,095,431 | 1,064,412 | 1,016,658 | 940,911 | 800,202 | 593,570 | 275,963
Accident Year 2008 | Property |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and claim expenses, net of reinsurance | 683,374 | 683,757 | 684,834 | 692,225 | 701,522 | 716,537 | 750,523 | 754,393 | 754,840 | 852,500
IBNR and ACR | 130 |  |  |  |  |  |  |  |  |
Cumulative paid claims and claim expenses, net of reinsurance | 685,874 | 685,229 | 683,821 | 681,988 | 678,266 | 664,217 | 631,204 | 541,278 | 392,800 | 247,123
Accident Year 2008 | Casualty and Specialty |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and claim expenses, net of reinsurance | 557,742 | 555,648 | 571,349 | 574,106 | 586,726 | 594,409 | 632,855 | 647,574 | 666,976 | 606,936
IBNR and ACR | 44,059 |  |  |  |  |  |  |  |  |
Cumulative paid claims and claim expenses, net of reinsurance | 478,773 | 459,920 | 439,466 | 413,443 | 386,146 | 352,441 | 309,707 | 258,924 | 200,770 | $ 28,840
Accident Year 2009 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and claim expenses, net of reinsurance | 531,523 | 528,693 | 533,597 | 542,653 | 563,850 | 589,136 | 628,671 | 645,608 | 711,042 |
IBNR and ACR | 23,021 |  |  |  |  |  |  |  |  |
Cumulative paid claims and claim expenses, net of reinsurance | 476,298 | 468,759 | 462,407 | 440,320 | 400,991 | 367,996 | 323,183 | 271,031 | 96,574 |
Accident Year 2009 | Property |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and claim expenses, net of reinsurance | 138,732 | 139,167 | 138,948 | 139,676 | 138,899 | 143,232 | 149,830 | 169,191 | 224,928 |
IBNR and ACR | 238 |  |  |  |  |  |  |  |  |
Cumulative paid claims and claim expenses, net of reinsurance | 137,775 | 137,783 | 136,806 | 135,756 | 131,136 | 125,021 | 117,045 | 102,402 | 56,244 |
Accident Year 2009 | Casualty and Specialty |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and claim expenses, net of reinsurance | 392,791 | 389,526 | 394,649 | 402,977 | 424,951 | 445,904 | 478,841 | 476,417 | 486,114 |
IBNR and ACR | 22,783 |  |  |  |  |  |  |  |  |
Cumulative paid claims and claim expenses, net of reinsurance | 338,523 | 330,976 | 325,601 | 304,564 | 269,855 | 242,975 | 206,138 | 168,629 | $ 40,330 |
Accident Year 2010 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and claim expenses, net of reinsurance | 894,050 | 895,201 | 886,997 | 896,219 | 897,148 | 927,941 | 975,846 | 1,016,922 |  |
IBNR and ACR | 76,527 |  |  |  |  |  |  |  |  |
Cumulative paid claims and claim expenses, net of reinsurance | 764,337 | 737,478 | 644,975 | 572,829 | 516,321 | 444,618 | 325,377 | 131,436 |  |
Accident Year 2010 | Property |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and claim expenses, net of reinsurance | 592,679 | 590,023 | 580,084 | 576,079 | 555,419 | 550,608 | 584,878 | 632,510 |  |
IBNR and ACR | 43,479 |  |  |  |  |  |  |  |  |
Cumulative paid claims and claim expenses, net of reinsurance | 506,297 | 496,324 | 417,446 | 389,954 | 352,293 | 308,198 | 226,433 | 96,570 |  |
Accident Year 2010 | Casualty and Specialty |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and claim expenses, net of reinsurance | 301,371 | 305,178 | 306,913 | 320,140 | 341,729 | 377,333 | 390,968 | 384,412 |  |
IBNR and ACR | 33,048 |  |  |  |  |  |  |  |  |
Cumulative paid claims and claim expenses, net of reinsurance | 258,040 | 241,154 | 227,529 | 182,875 | 164,028 | 136,420 | 98,944 | $ 34,866 |  |
Accident Year 2011 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and claim expenses, net of reinsurance | 1,361,430 | 1,375,169 | 1,413,256 | 1,441,370 | 1,523,332 | 1,604,273 | 1,670,722 |  |  |
IBNR and ACR | 89,094 |  |  |  |  |  |  |  |  |
Cumulative paid claims and claim expenses, net of reinsurance | 1,230,285 | 1,202,703 | 1,158,233 | 1,067,386 | 909,176 | 550,668 | 259,261 |  |  |
Accident Year 2011 | Property |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and claim expenses, net of reinsurance | 1,063,035 | 1,066,200 | 1,098,124 | 1,118,651 | 1,170,141 | 1,221,811 | 1,286,890 |  |  |
IBNR and ACR | 41,910 |  |  |  |  |  |  |  |  |
Cumulative paid claims and claim expenses, net of reinsurance | 992,272 | 978,298 | 951,725 | 891,928 | 763,384 | 437,082 | 211,151 |  |  |
Accident Year 2011 | Casualty and Specialty |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and claim expenses, net of reinsurance | 298,395 | 308,969 | 315,132 | 322,719 | 353,191 | 382,462 | 383,832 |  |  |
IBNR and ACR | 47,184 |  |  |  |  |  |  |  |  |
Cumulative paid claims and claim expenses, net of reinsurance | 238,013 | 224,405 | 206,508 | 175,458 | 145,792 | 113,586 | $ 48,110 |  |  |
Accident Year 2012 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and claim expenses, net of reinsurance | 658,180 | 655,602 | 683,784 | 710,123 | 773,367 | 867,675 |  |  |  |
IBNR and ACR | 94,086 |  |  |  |  |  |  |  |  |
Cumulative paid claims and claim expenses, net of reinsurance | 527,153 | 462,253 | 418,697 | 358,789 | 267,650 | 166,907 |  |  |  |
Accident Year 2012 | Property |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and claim expenses, net of reinsurance | 265,058 | 276,236 | 294,400 | 312,216 | 345,168 | 438,548 |  |  |  |
IBNR and ACR | 24,297 |  |  |  |  |  |  |  |  |
Cumulative paid claims and claim expenses, net of reinsurance | 231,272 | 219,069 | 209,059 | 189,555 | 145,326 | 100,067 |  |  |  |
Accident Year 2012 | Casualty and Specialty |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and claim expenses, net of reinsurance | 393,122 | 379,366 | 389,384 | 397,907 | 428,199 | 429,127 |  |  |  |
IBNR and ACR | 69,789 |  |  |  |  |  |  |  |  |
Cumulative paid claims and claim expenses, net of reinsurance | 295,881 | 243,184 | 209,638 | 169,234 | 122,324 | $ 66,840 |  |  |  |
Accident Year 2013 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and claim expenses, net of reinsurance | 450,984 | 477,294 | 518,266 | 564,791 | 625,081 |  |  |  |  |
IBNR and ACR | 86,709 |  |  |  |  |  |  |  |  |
Cumulative paid claims and claim expenses, net of reinsurance | 330,184 | 292,542 | 246,582 | 181,738 | 87,837 |  |  |  |  |
Accident Year 2013 | Property |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and claim expenses, net of reinsurance | 143,823 | 155,384 | 177,533 | 200,105 | 230,355 |  |  |  |  |
IBNR and ACR | 5,264 |  |  |  |  |  |  |  |  |
Cumulative paid claims and claim expenses, net of reinsurance | 135,499 | 131,293 | 120,117 | 94,776 | 49,805 |  |  |  |  |
Accident Year 2013 | Casualty and Specialty |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and claim expenses, net of reinsurance | 307,161 | 321,910 | 340,733 | 364,686 | 394,726 |  |  |  |  |
IBNR and ACR | 81,445 |  |  |  |  |  |  |  |  |
Cumulative paid claims and claim expenses, net of reinsurance | 194,685 | 161,249 | 126,465 | 86,962 | $ 38,032 |  |  |  |  |
Accident Year 2014 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and claim expenses, net of reinsurance | 586,559 | 605,648 | 617,617 | 664,603 |  |  |  |  |  |
IBNR and ACR | 83,121 |  |  |  |  |  |  |  |  |
Cumulative paid claims and claim expenses, net of reinsurance | 319,540 | 270,380 | 201,873 | 111,451 |  |  |  |  |  |
Accident Year 2014 | Property |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and claim expenses, net of reinsurance | 143,419 | 147,610 | 155,335 | 184,076 |  |  |  |  |  |
IBNR and ACR | 8,018 |  |  |  |  |  |  |  |  |
Cumulative paid claims and claim expenses, net of reinsurance | 124,533 | 119,473 | 96,667 | 55,365 |  |  |  |  |  |
Accident Year 2014 | Casualty and Specialty |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and claim expenses, net of reinsurance | 443,140 | 458,038 | 462,282 | 480,527 |  |  |  |  |  |
IBNR and ACR | 75,103 |  |  |  |  |  |  |  |  |
Cumulative paid claims and claim expenses, net of reinsurance | 195,007 | 150,907 | 105,206 | $ 56,086 |  |  |  |  |  |
Accident Year 2015 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and claim expenses, net of reinsurance | 634,948 | 631,559 | 642,977 |  |  |  |  |  |  |
IBNR and ACR | 240,331 |  |  |  |  |  |  |  |  |
Cumulative paid claims and claim expenses, net of reinsurance | 289,630 | 195,140 | 96,141 |  |  |  |  |  |  |
Accident Year 2015 | Property |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and claim expenses, net of reinsurance | 177,296 | 196,094 | 227,093 |  |  |  |  |  |  |
IBNR and ACR | 32,432 |  |  |  |  |  |  |  |  |
Cumulative paid claims and claim expenses, net of reinsurance | 128,434 | 109,386 | 62,358 |  |  |  |  |  |  |
Accident Year 2015 | Casualty and Specialty |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and claim expenses, net of reinsurance | 457,652 | 435,465 | 415,884 |  |  |  |  |  |  |
IBNR and ACR | 207,899 |  |  |  |  |  |  |  |  |
Cumulative paid claims and claim expenses, net of reinsurance | 161,196 | 85,754 | $ 33,783 |  |  |  |  |  |  |
Accident Year 2016 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and claim expenses, net of reinsurance | 691,472 | 684,960 |  |  |  |  |  |  |  |
IBNR and ACR | 358,843 |  |  |  |  |  |  |  |  |
Cumulative paid claims and claim expenses, net of reinsurance | 219,416 | 79,943 |  |  |  |  |  |  |  |
Accident Year 2016 | Property |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and claim expenses, net of reinsurance | 256,759 | 254,595 |  |  |  |  |  |  |  |
IBNR and ACR | 91,193 |  |  |  |  |  |  |  |  |
Cumulative paid claims and claim expenses, net of reinsurance | 119,977 | 47,716 |  |  |  |  |  |  |  |
Accident Year 2016 | Casualty and Specialty |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and claim expenses, net of reinsurance | 434,713 | 430,365 |  |  |  |  |  |  |  |
IBNR and ACR | 267,650 |  |  |  |  |  |  |  |  |
Cumulative paid claims and claim expenses, net of reinsurance | 99,439 | $ 32,227 |  |  |  |  |  |  |  |
Accident Year 2017 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and claim expenses, net of reinsurance | 1,901,309 |  |  |  |  |  |  |  |  |
IBNR and ACR | 1,334,903 |  |  |  |  |  |  |  |  |
Cumulative paid claims and claim expenses, net of reinsurance | 450,886 |  |  |  |  |  |  |  |  |
Accident Year 2017 | Property |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and claim expenses, net of reinsurance | 1,345,006 |  |  |  |  |  |  |  |  |
IBNR and ACR | 857,673 |  |  |  |  |  |  |  |  |
Cumulative paid claims and claim expenses, net of reinsurance | 412,477 |  |  |  |  |  |  |  |  |
Accident Year 2017 | Casualty and Specialty |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and claim expenses, net of reinsurance | 556,303 |  |  |  |  |  |  |  |  |
IBNR and ACR | 477,230 |  |  |  |  |  |  |  |  |
Cumulative paid claims and claim expenses, net of reinsurance | $ 38,409 |  |  |  |  |  |  |  |  |

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 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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4B -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=65670685&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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4D -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=65670685&loc=SL65671317-158438 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4E -URI http://asc.fasb.org/extlink&oid=65670685&loc=SL65671321-158438 + 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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4B -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=65670685&loc=SL65671311-158438 + 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. Excludes unallocated claim adjustment expense. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4B -URI http://asc.fasb.org/extlink&oid=65670685&loc=SL65671311-158438 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4C -URI http://asc.fasb.org/extlink&oid=65670685&loc=SL65671315-158438 + 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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4B -URI http://asc.fasb.org/extlink&oid=65670685&loc=SL65671311-158438 + Details
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- Definition Amount of reconciling item for the reconciliation of the liability in the disclosure of claims development (in excess of) less than the liability for unpaid claims and claims adjustment expense for short-duration insurance contracts, classified as other. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4C -URI http://asc.fasb.org/extlink&oid=65670685&loc=SL65671315-158438 + Details
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- Definition Amount of liability for unallocated claim adjustment expense for short-duration insurance contracts. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4C -URI http://asc.fasb.org/extlink&oid=65670685&loc=SL65671315-158438 + Details
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