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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, 2024 USD ($)	Dec. 31, 2023 USD ($)	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 ($)	Dec. 31, 2015 USD ($)
Claims Development [Line Items]
Total liabilities for unpaid losses and loss adjustment expenses, net of reinsurance	$ 15,087,702,000
Insurance [Member]
Claims Development [Line Items]
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance	27,539,900,000
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	16,302,700,000
All outstanding liabilities for unpaid losses and loss adjustment expenses before 2014, net of reinsurance	313,300,000
Total liabilities for unpaid losses and loss adjustment expenses, net of reinsurance	11,550,516,000
Insurance [Member] | 2015 Accident Year [Member]
Claims Development [Line Items]
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance	1,457,100,000	$ 1,460,000,000	$ 1,440,900,000	$ 1,450,600,000	$ 1,454,700,000	$ 1,486,500,000	$ 1,517,000,000	$ 1,571,000,000	$ 1,693,800,000	$ 1,767,500,000
Total of incurred-but-not-reported liabilities, net of reinsurance	$ 58,000,000.0
Cumulative number of reported claims	88,000
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	$ 1,344,700,000	1,327,300,000	1,291,700,000	1,259,200,000	1,208,400,000	1,141,300,000	1,032,900,000	870,200,000	660,000,000.0	320,700,000
Insurance [Member] | 2016 Accident Year [Member]
Claims Development [Line Items]
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance	1,718,100,000	1,732,900,000	1,678,400,000	1,667,400,000	1,674,700,000	1,700,300,000	1,753,600,000	1,852,800,000	1,856,300,000
Total of incurred-but-not-reported liabilities, net of reinsurance	$ 77,200,000
Cumulative number of reported claims	100,000
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	$ 1,529,800,000	1,479,600,000	1,425,500,000	1,350,300,000	1,262,800,000	1,158,200,000	973,700,000	746,200,000	369,800,000
Insurance [Member] | 2017 Accident Year [Member]
Claims Development [Line Items]
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance	2,152,300,000	2,111,200,000	2,023,900,000	2,004,500,000	1,998,700,000	2,059,300,000	2,176,700,000	2,309,000,000
Total of incurred-but-not-reported liabilities, net of reinsurance	$ 124,600,000
Cumulative number of reported claims	140,000
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	$ 1,909,500,000	1,839,600,000	1,739,800,000	1,619,200,000	1,462,000,000	1,274,300,000	982,700,000	434,900,000
Insurance [Member] | 2018 Accident Year [Member]
Claims Development [Line Items]
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance	2,417,100,000	2,331,600,000	2,151,200,000	2,058,800,000	2,199,200,000	2,320,800,000	2,429,000,000
Total of incurred-but-not-reported liabilities, net of reinsurance	$ 177,800,000
Cumulative number of reported claims	193,000
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	$ 2,017,400,000	1,856,500,000	1,671,600,000	1,494,900,000	1,281,900,000	1,017,400,000	490,700,000
Insurance [Member] | 2019 Accident Year [Member]
Claims Development [Line Items]
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance	2,526,800,000	2,471,700,000	2,293,700,000	2,239,700,000	2,402,200,000	2,552,400,000
Total of incurred-but-not-reported liabilities, net of reinsurance	$ 206,900,000
Cumulative number of reported claims	228,000
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	$ 2,104,400,000	1,910,900,000	1,576,400,000	1,262,900,000	991,100,000	524,100,000
Insurance [Member] | 2020 Accident Year [Member]
Claims Development [Line Items]
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance	2,771,600,000	2,858,500,000	2,914,200,000	2,972,100,000	3,185,100,000
Total of incurred-but-not-reported liabilities, net of reinsurance	$ 434,300,000
Cumulative number of reported claims	183,000
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	$ 2,033,800,000	1,772,500,000	1,515,700,000	1,148,500,000	568,100,000
Insurance [Member] | 2021 Accident Year [Member]
Claims Development [Line Items]
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance	2,720,100,000	2,755,400,000	2,907,100,000	3,083,500,000
Total of incurred-but-not-reported liabilities, net of reinsurance	$ 531,000,000.0
Cumulative number of reported claims	149,000
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	$ 1,751,100,000	1,357,500,000	985,800,000	473,400,000
Insurance [Member] | 2022 Accident Year [Member]
Claims Development [Line Items]
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance	3,342,600,000	3,365,500,000	3,746,800,000
Total of incurred-but-not-reported liabilities, net of reinsurance	$ 1,098,300,000
Cumulative number of reported claims	159,000
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	$ 1,564,100,000	1,059,900,000	484,500,000
Insurance [Member] | 2023 Accident Year [Member]
Claims Development [Line Items]
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance	3,901,700,000	4,435,900,000
Total of incurred-but-not-reported liabilities, net of reinsurance	$ 1,802,500,000
Cumulative number of reported claims	169,000
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	$ 1,396,600,000	596,900,000
Insurance [Member] | 2024 Accident Year [Member]
Claims Development [Line Items]
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance	4,532,500,000
Total of incurred-but-not-reported liabilities, net of reinsurance	$ 3,271,700,000
Cumulative number of reported claims	149,000
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	$ 651,300,000
Reinsurance [Member]
Claims Development [Line Items]
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance	7,017,100,000
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	3,847,700,000
All outstanding liabilities for unpaid losses and loss adjustment expenses before 2014, net of reinsurance	273,300,000
Total liabilities for unpaid losses and loss adjustment expenses, net of reinsurance	3,442,663,000
Reinsurance [Member] | 2015 Accident Year [Member]
Claims Development [Line Items]
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance	477,700,000	500,000,000.0	488,200,000	487,800,000	499,900,000	504,800,000	516,000,000.0	524,600,000	508,800,000	523,000,000.0
Total of incurred-but-not-reported liabilities, net of reinsurance	56,600,000
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	388,600,000	375,600,000	359,500,000	342,800,000	323,200,000	298,600,000	251,600,000	201,000,000.0	130,300,000	$ 63,300,000
Reinsurance [Member] | 2016 Accident Year [Member]
Claims Development [Line Items]
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance	556,500,000	573,300,000	551,500,000	548,100,000	523,400,000	513,400,000	515,400,000	515,200,000	506,600,000
Total of incurred-but-not-reported liabilities, net of reinsurance	30,100,000
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	475,700,000	451,600,000	418,000,000.0	377,900,000	344,400,000	292,700,000	236,400,000	166,700,000	$ 78,600,000
Reinsurance [Member] | 2017 Accident Year [Member]
Claims Development [Line Items]
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance	931,300,000	931,000,000.0	919,700,000	907,300,000	934,900,000	933,200,000	927,600,000	894,400,000
Total of incurred-but-not-reported liabilities, net of reinsurance	44,200,000
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	796,000,000.0	747,600,000	692,800,000	623,800,000	560,400,000	478,600,000	357,500,000	$ 157,100,000
Reinsurance [Member] | 2018 Accident Year [Member]
Claims Development [Line Items]
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance	790,100,000	821,000,000.0	758,100,000	762,500,000	757,400,000	764,900,000	743,000,000.0
Total of incurred-but-not-reported liabilities, net of reinsurance	82,200,000
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	613,900,000	543,600,000	474,400,000	404,400,000	334,900,000	235,100,000	$ 86,900,000
Reinsurance [Member] | 2019 Accident Year [Member]
Claims Development [Line Items]
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance	730,400,000	719,000,000.0	691,000,000.0	683,100,000	668,400,000	654,400,000
Total of incurred-but-not-reported liabilities, net of reinsurance	129,100,000
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	507,700,000	435,600,000	351,800,000	261,300,000	169,100,000	$ 53,200,000
Reinsurance [Member] | 2020 Accident Year [Member]
Claims Development [Line Items]
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance	765,200,000	762,800,000	741,000,000.0	730,500,000	678,100,000
Total of incurred-but-not-reported liabilities, net of reinsurance	230,100,000
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	463,000,000.0	381,800,000	308,700,000	202,900,000	$ 93,200,000
Reinsurance [Member] | 2021 Accident Year [Member]
Claims Development [Line Items]
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance	753,900,000	754,700,000	733,600,000	731,900,000
Total of incurred-but-not-reported liabilities, net of reinsurance	303,800,000
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	361,000,000.0	281,600,000	187,100,000	$ 79,200,000
Reinsurance [Member] | 2022 Accident Year [Member]
Claims Development [Line Items]
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance	616,400,000	601,900,000	661,100,000
Total of incurred-but-not-reported liabilities, net of reinsurance	413,400,000
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	138,400,000	66,600,000	$ 24,100,000
Reinsurance [Member] | 2023 Accident Year [Member]
Claims Development [Line Items]
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance	665,300,000	658,300,000
Total of incurred-but-not-reported liabilities, net of reinsurance	506,700,000
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	79,500,000	$ 27,100,000
Reinsurance [Member] | 2024 Accident Year [Member]
Claims Development [Line Items]
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance	730,300,000
Total of incurred-but-not-reported liabilities, net of reinsurance	674,300,000
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	$ 23,900,000

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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.

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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.

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