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Unpaid Losses And Loss Adjustment Expenses (Ultimate Incurred Losses And Cumulative Paid Losses And Allocated Loss Adjustment Expenses, Net Of Reinsurance) (Details) | Dec. 31, 2023 USD ($) | Dec. 31, 2022 USD ($) | Dec. 31, 2021 USD ($) | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($)
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Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Total liabilities for unpaid losses and loss adjustment expenses, net of reinsurance | $ 14,284,815,000 |  |  |  |  |  |  |  |  |
Insurance [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance | 25,087,100,000 |  |  |  |  |  |  |  |  |
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance | 14,639,500,000 |  |  |  |  |  |  |  |  |
All outstanding liabilities for unpaid losses and loss adjustment expenses before 2014, net of reinsurance | 258,200,000 |  |  |  |  |  |  |  |  |
Total liabilities for unpaid losses and loss adjustment expenses, net of reinsurance | 10,705,776,000 |  |  |  |  |  |  |  |  |
Insurance [Member] | 2014 Accident Year [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance | 1,431,500,000 | $ 1,446,300,000 | $ 1,466,200,000 | $ 1,469,300,000 | $ 1,496,000,000 | $ 1,517,200,000 | $ 1,565,500,000 | $ 1,622,700,000 | $ 1,690,300,000 | $ 1,859,000,000
Total of incurred-but-not-reported liabilities, net of reinsurance | $ 30,100,000 |  |  |  |  |  |  |  |  |
Cumulative number of reported claims | 86,000 |  |  |  |  |  |  |  |  |
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance | $ 1,365,400,000 | 1,347,100,000 | 1,315,700,000 | 1,290,000,000 | 1,246,900,000 | 1,163,300,000 | 1,058,200,000 | 891,000,000.0 | 656,100,000 | 331,000,000.0
Insurance [Member] | 2015 Accident Year [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance | 1,470,700,000 | 1,450,400,000 | 1,460,600,000 | 1,464,900,000 | 1,497,200,000 | 1,527,900,000 | 1,582,000,000 | 1,704,800,000 | 1,778,100,000 |
Total of incurred-but-not-reported liabilities, net of reinsurance | $ 67,800,000 |  |  |  |  |  |  |  |  |
Cumulative number of reported claims | 90,000 |  |  |  |  |  |  |  |  |
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance | $ 1,335,300,000 | 1,299,400,000 | 1,266,800,000 | 1,215,000,000 | 1,147,100,000 | 1,037,700,000 | 874,000,000.0 | 663,000,000.0 | 321,800,000 |
Insurance [Member] | 2016 Accident Year [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance | 1,742,500,000 | 1,687,900,000 | 1,676,200,000 | 1,683,400,000 | 1,710,100,000 | 1,763,400,000 | 1,862,700,000 | 1,866,100,000 |  |
Total of incurred-but-not-reported liabilities, net of reinsurance | $ 123,900,000 |  |  |  |  |  |  |  |  |
Cumulative number of reported claims | 100,000 |  |  |  |  |  |  |  |  |
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance | $ 1,488,300,000 | 1,434,000,000 | 1,357,600,000 | 1,269,700,000 | 1,164,500,000 | 979,000,000.0 | 750,000,000.0 | 371,300,000 |  |
Insurance [Member] | 2017 Accident Year [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance | 2,122,900,000 | 2,035,400,000 | 2,016,200,000 | 2,009,300,000 | 2,071,000,000 | 2,190,600,000 | 2,321,400,000 |  |  |
Total of incurred-but-not-reported liabilities, net of reinsurance | $ 138,200,000 |  |  |  |  |  |  |  |  |
Cumulative number of reported claims | 138,000 |  |  |  |  |  |  |  |  |
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance | $ 1,850,100,000 | 1,749,700,000 | 1,628,100,000 | 1,469,800,000 | 1,281,200,000 | 988,200,000 | 437,200,000 |  |  |
Insurance [Member] | 2018 Accident Year [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance | 2,347,400,000 | 2,166,100,000 | 2,074,100,000 | 2,215,000,000 | 2,339,200,000 | 2,444,600,000 |  |  |  |
Total of incurred-but-not-reported liabilities, net of reinsurance | $ 216,200,000 |  |  |  |  |  |  |  |  |
Cumulative number of reported claims | 191,000 |  |  |  |  |  |  |  |  |
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance | $ 1,871,400,000 | 1,684,400,000 | 1,507,000,000 | 1,292,900,000 | 1,023,800,000 | 494,100,000 |  |  |  |
Insurance [Member] | 2019 Accident Year [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance | 2,486,800,000 | 2,308,400,000 | 2,257,800,000 | 2,421,100,000 | 2,569,400,000 |  |  |  |  |
Total of incurred-but-not-reported liabilities, net of reinsurance | $ 296,400,000 |  |  |  |  |  |  |  |  |
Cumulative number of reported claims | 227,000 |  |  |  |  |  |  |  |  |
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance | $ 1,920,300,000 | 1,584,000,000 | 1,269,800,000 | 995,800,000 | 526,000,000.0 |  |  |  |  |
Insurance [Member] | 2020 Accident Year [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance | 2,867,600,000 | 2,928,000,000 | 2,987,600,000 | 3,202,300,000 |  |  |  |  |  |
Total of incurred-but-not-reported liabilities, net of reinsurance | $ 770,700,000 |  |  |  |  |  |  |  |  |
Cumulative number of reported claims | 182,000 |  |  |  |  |  |  |  |  |
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance | $ 1,783,200,000 | 1,525,000,000 | 1,155,200,000 | 571,200,000 |  |  |  |  |  |
Insurance [Member] | 2021 Accident Year [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance | 2,770,900,000 | 2,926,700,000 | 3,103,700,000 |  |  |  |  |  |  |
Total of incurred-but-not-reported liabilities, net of reinsurance | $ 986,700,000 |  |  |  |  |  |  |  |  |
Cumulative number of reported claims | 143,000 |  |  |  |  |  |  |  |  |
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance | $ 1,363,000,000 | 989,700,000 | 475,000,000.0 |  |  |  |  |  |  |
Insurance [Member] | 2022 Accident Year [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance | 3,385,500,000 | 3,767,700,000 |  |  |  |  |  |  |  |
Total of incurred-but-not-reported liabilities, net of reinsurance | $ 1,765,500,000 |  |  |  |  |  |  |  |  |
Cumulative number of reported claims | 149,000 |  |  |  |  |  |  |  |  |
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance | $ 1,063,600,000 | 486,200,000 |  |  |  |  |  |  |  |
Insurance [Member] | 2023 Accident Year [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance | 4,461,300,000 |  |  |  |  |  |  |  |  |
Total of incurred-but-not-reported liabilities, net of reinsurance | $ 3,324,300,000 |  |  |  |  |  |  |  |  |
Cumulative number of reported claims | 135,000 |  |  |  |  |  |  |  |  |
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance | $ 598,900,000 |  |  |  |  |  |  |  |  |
Reinsurance [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance | 6,841,400,000 |  |  |  |  |  |  |  |  |
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance | 3,739,100,000 |  |  |  |  |  |  |  |  |
All outstanding liabilities for unpaid losses and loss adjustment expenses before 2014, net of reinsurance | 210,100,000 |  |  |  |  |  |  |  |  |
Total liabilities for unpaid losses and loss adjustment expenses, net of reinsurance | 3,312,405,000 |  |  |  |  |  |  |  |  |
Reinsurance [Member] | 2014 Accident Year [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance | 477,100,000 | 508,100,000 | 512,700,000 | 511,300,000 | 526,900,000 | 550,400,000 | 571,800,000 | 529,500,000 | 557,100,000 | 572,600,000
Total of incurred-but-not-reported liabilities, net of reinsurance | 46,000,000.0 |  |  |  |  |  |  |  |  |
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance | 398,100,000 | 384,300,000 | 373,900,000 | 357,300,000 | 339,600,000 | 306,000,000.0 | 269,500,000 | 222,500,000 | 156,400,000 | $ 97,600,000
Reinsurance [Member] | 2015 Accident Year [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance | 503,200,000 | 490,700,000 | 490,500,000 | 502,800,000 | 508,100,000 | 519,300,000 | 528,100,000 | 511,200,000 | 525,300,000 |
Total of incurred-but-not-reported liabilities, net of reinsurance | 81,900,000 |  |  |  |  |  |  |  |  |
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance | 380,200,000 | 364,000,000.0 | 347,100,000 | 327,200,000 | 302,300,000 | 255,000,000.0 | 204,100,000 | 132,000,000.0 | $ 63,700,000 |
Reinsurance [Member] | 2016 Accident Year [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance | 580,300,000 | 558,300,000 | 554,800,000 | 531,100,000 | 521,300,000 | 523,000,000.0 | 523,600,000 | 513,800,000 |  |
Total of incurred-but-not-reported liabilities, net of reinsurance | 62,700,000 |  |  |  |  |  |  |  |  |
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance | 455,700,000 | 421,900,000 | 381,600,000 | 347,800,000 | 295,800,000 | 239,300,000 | 168,800,000 | $ 79,300,000 |  |
Reinsurance [Member] | 2017 Accident Year [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance | 934,800,000 | 923,000,000.0 | 911,000,000.0 | 939,000,000.0 | 937,800,000 | 932,500,000 | 899,500,000 |  |  |
Total of incurred-but-not-reported liabilities, net of reinsurance | 80,300,000 |  |  |  |  |  |  |  |  |
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance | 749,900,000 | 694,900,000 | 625,700,000 | 562,100,000 | 480,000,000.0 | 358,500,000 | $ 157,600,000 |  |  |
Reinsurance [Member] | 2018 Accident Year [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance | 829,300,000 | 765,700,000 | 770,000,000.0 | 765,500,000 | 772,900,000 | 748,100,000 |  |  |  |
Total of incurred-but-not-reported liabilities, net of reinsurance | 157,800,000 |  |  |  |  |  |  |  |  |
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance | 551,000,000.0 | 481,500,000 | 411,300,000 | 341,300,000 | 240,500,000 | $ 87,100,000 |  |  |  |
Reinsurance [Member] | 2019 Accident Year [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance | 726,100,000 | 700,200,000 | 689,200,000 | 675,500,000 | 661,600,000 |  |  |  |  |
Total of incurred-but-not-reported liabilities, net of reinsurance | 174,200,000 |  |  |  |  |  |  |  |  |
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance | 441,500,000 | 357,400,000 | 266,300,000 | 172,300,000 | $ 53,600,000 |  |  |  |  |
Reinsurance [Member] | 2020 Accident Year [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance | 764,700,000 | 745,200,000 | 735,000,000.0 | 681,300,000 |  |  |  |  |  |
Total of incurred-but-not-reported liabilities, net of reinsurance | 286,600,000 |  |  |  |  |  |  |  |  |
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance | 384,500,000 | 311,100,000 | 204,600,000 | $ 94,000,000.0 |  |  |  |  |  |
Reinsurance [Member] | 2021 Accident Year [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance | 760,800,000 | 739,700,000 | 738,600,000 |  |  |  |  |  |  |
Total of incurred-but-not-reported liabilities, net of reinsurance | 379,300,000 |  |  |  |  |  |  |  |  |
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance | 284,000,000.0 | 188,400,000 | $ 79,600,000 |  |  |  |  |  |  |
Reinsurance [Member] | 2022 Accident Year [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance | 604,500,000 | 664,200,000 |  |  |  |  |  |  |  |
Total of incurred-but-not-reported liabilities, net of reinsurance | 483,000,000.0 |  |  |  |  |  |  |  |  |
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance | 66,900,000 | $ 24,200,000 |  |  |  |  |  |  |  |
Reinsurance [Member] | 2023 Accident Year [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance | 660,600,000 |  |  |  |  |  |  |  |  |
Total of incurred-but-not-reported liabilities, net of reinsurance | 607,200,000 |  |  |  |  |  |  |  |  |
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance | $ 27,300,000 |  |  |  |  |  |  |  |  |

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