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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 ($)	Dec. 31, 2014 USD ($)
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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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.

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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 incurred claims and allocated claim adjustment expense used in claims development 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. 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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Cumulative number of reported claims for short-duration insurance contracts.

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