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Reserves for Losses and Adjustment Expenses Short-duration Insurance Contracts, Claims by Accident Year (Details)
Dec. 31, 2024
USD ($)
Integer
Dec. 31, 2023
USD ($)
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]                    
Liability for claims and claim adjustment expenses, net of reinsurance $ 4,116,600,000 $ 3,629,600,000                
Property Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 1,816,400,000                  
Cumulative paid claims and allocated claim adjustment expense, Net 1,547,500,000                  
All outstanding liabilities for 2015 and subsequent years, net of reinsurance 268,900,000                  
All outstanding liabilities before 2015, net of reinsurance 0                  
Liability for claims and claim adjustment expenses, net of reinsurance 268,900,000 270,800,000                
Casualty Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 1,477,200,000                  
Cumulative paid claims and allocated claim adjustment expense, Net 721,200,000                  
All outstanding liabilities for 2015 and subsequent years, net of reinsurance 756,000,000.0                  
All outstanding liabilities before 2015, net of reinsurance 0                  
Liability for claims and claim adjustment expenses, net of reinsurance 756,000,000.0 636,500,000                
Marine, Aviation and Energy Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 1,434,400,000                  
Cumulative paid claims and allocated claim adjustment expense, Net 1,213,100,000                  
All outstanding liabilities for 2015 and subsequent years, net of reinsurance 221,300,000                  
All outstanding liabilities before 2015, net of reinsurance 0                  
Liability for claims and claim adjustment expenses, net of reinsurance 221,300,000 194,500,000                
Financial and Professional Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 2,338,700,000                  
Cumulative paid claims and allocated claim adjustment expense, Net 1,357,400,000                  
All outstanding liabilities for 2015 and subsequent years, net of reinsurance 981,300,000                  
All outstanding liabilities before 2015, net of reinsurance 0                  
Liability for claims and claim adjustment expenses, net of reinsurance 981,300,000 837,600,000                
Property Catastrophe and Other Property Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 2,925,200,000                  
Cumulative paid claims and allocated claim adjustment expense, Net 2,364,600,000                  
All outstanding liabilities for 2015 and subsequent years, net of reinsurance 560,600,000                  
All outstanding liabilities before 2015, net of reinsurance 0                  
Liability for claims and claim adjustment expenses, net of reinsurance 560,600,000 562,300,000                
Casualty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 1,691,100,000                  
Cumulative paid claims and allocated claim adjustment expense, Net 805,200,000                  
All outstanding liabilities for 2015 and subsequent years, net of reinsurance 885,900,000                  
All outstanding liabilities before 2015, net of reinsurance 0                  
Liability for claims and claim adjustment expenses, net of reinsurance 885,900,000 733,600,000                
Specialty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 2,339,100,000                  
Cumulative paid claims and allocated claim adjustment expense, Net 1,896,500,000                  
All outstanding liabilities for 2015 and subsequent years, net of reinsurance 442,600,000                  
All outstanding liabilities before 2015, net of reinsurance 0                  
Liability for claims and claim adjustment expenses, net of reinsurance 442,600,000 394,300,000                
Accident Year 2015 | Property Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 192,300,000 192,300,000 $ 192,300,000 $ 192,300,000 $ 196,300,000 $ 199,400,000 $ 198,800,000 $ 196,500,000 $ 201,700,000 $ 236,000,000.0
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 11,558                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 192,300,000 192,300,000 192,300,000 192,300,000 191,300,000 192,600,000 176,400,000 167,400,000 140,200,000 56,700,000
Accident Year 2015 | Casualty Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 178,900,000 178,900,000 178,900,000 255,600,000 230,300,000 232,000,000.0 199,400,000 182,300,000 219,800,000 199,800,000
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 4,843                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 178,900,000 178,900,000 178,900,000 178,900,000 165,000,000.0 136,200,000 91,000,000.0 55,700,000 16,600,000 3,100,000
Accident Year 2015 | Marine, Aviation and Energy Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 266,000,000.0 266,000,000.0 266,000,000.0 317,200,000 309,900,000 307,200,000 283,600,000 278,700,000 296,000,000.0 294,200,000
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 4,079                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 266,000,000.0 266,000,000.0 266,000,000.0 267,500,000 255,800,000 220,100,000 192,200,000 172,600,000 122,200,000 44,500,000
Accident Year 2015 | Financial and Professional Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 144,400,000 144,400,000 144,400,000 195,800,000 183,000,000.0 188,000,000.0 187,000,000.0 183,100,000 173,000,000.0 171,700,000
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 1,082                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 144,400,000 144,400,000 144,400,000 150,100,000 137,700,000 108,800,000 88,700,000 69,700,000 43,200,000 13,600,000
Accident Year 2015 | Property Catastrophe and Other Property Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 151,900,000 151,900,000 151,900,000 173,000,000.0 166,900,000 166,800,000 151,400,000 171,500,000 181,100,000 206,700,000
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 1,045                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 151,900,000 151,900,000 151,900,000 155,600,000 154,200,000 151,900,000 135,000,000.0 122,000,000.0 92,400,000 35,600,000
Accident Year 2015 | Casualty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 115,200,000 115,200,000 115,200,000 192,200,000 201,900,000 205,500,000 207,600,000 204,900,000 195,200,000 188,900,000
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 2,090                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 115,200,000 115,200,000 115,200,000 115,200,000 106,800,000 88,000,000.0 64,400,000 37,500,000 17,400,000 3,300,000
Accident Year 2015 | Specialty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 129,200,000 129,200,000 129,200,000 147,900,000 148,100,000 152,500,000 154,000,000.0 159,300,000 164,500,000 161,600,000
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 775                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 129,200,000 129,200,000 129,200,000 129,900,000 130,100,000 127,100,000 118,600,000 101,600,000 54,600,000 $ 17,200,000
Accident Year 2016 | Property Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 232,600,000 232,600,000 232,600,000 233,400,000 244,400,000 243,100,000 241,800,000 246,800,000 235,800,000  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 10,752                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 232,600,000 232,600,000 232,600,000 233,400,000 230,300,000 221,000,000.0 199,500,000 167,300,000 66,300,000  
Accident Year 2016 | Casualty Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 122,900,000 122,900,000 122,900,000 241,300,000 196,900,000 185,800,000 179,400,000 184,300,000 213,300,000  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 4,850                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 122,900,000 122,900,000 122,900,000 122,000,000.0 107,200,000 80,600,000 39,100,000 22,300,000 4,100,000  
Accident Year 2016 | Marine, Aviation and Energy Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 196,200,000 196,200,000 196,200,000 218,600,000 216,800,000 227,000,000.0 226,400,000 228,100,000 257,800,000  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 4,437                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 196,200,000 196,200,000 196,200,000 191,700,000 188,700,000 162,300,000 140,600,000 81,700,000 30,900,000  
Accident Year 2016 | Financial and Professional Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 133,000,000.0 133,000,000.0 133,000,000.0 184,300,000 182,800,000 199,600,000 213,500,000 209,100,000 188,100,000  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 1,244                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 133,000,000.0 133,000,000.0 133,000,000.0 129,200,000 124,600,000 128,700,000 100,700,000 70,600,000 14,800,000  
Accident Year 2016 | Property Catastrophe and Other Property Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 221,700,000 221,700,000 221,700,000 224,300,000 231,800,000 236,300,000 256,800,000 257,200,000 257,200,000  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 1,281                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 221,700,000 221,700,000 221,700,000 224,300,000 219,500,000 207,000,000.0 196,800,000 157,100,000 54,700,000  
Accident Year 2016 | Casualty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 135,200,000 135,200,000 135,200,000 246,600,000 255,800,000 248,400,000 238,300,000 238,700,000 226,600,000  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 2,268                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 135,200,000 135,200,000 135,200,000 135,400,000 123,800,000 94,200,000 62,400,000 32,600,000 8,700,000  
Accident Year 2016 | Specialty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 185,800,000 185,800,000 185,800,000 208,800,000 221,000,000.0 225,900,000 232,700,000 234,700,000 233,900,000  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 943                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 185,800,000 185,800,000 185,800,000 192,000,000.0 190,600,000 180,500,000 162,700,000 148,800,000 $ 57,900,000  
Accident Year 2017 | Property Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 180,400,000 180,400,000 180,400,000 272,100,000 250,100,000 248,800,000 255,600,000 292,500,000    
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 9,711                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 180,400,000 180,400,000 180,400,000 233,600,000 239,700,000 219,000,000.0 186,400,000 95,500,000    
Accident Year 2017 | Casualty Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 60,200,000 60,200,000 60,200,000 213,300,000 192,500,000 175,100,000 171,400,000 177,900,000    
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 5,536                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 60,200,000 60,200,000 60,200,000 88,100,000 94,800,000 51,800,000 22,500,000 3,500,000    
Accident Year 2017 | Marine, Aviation and Energy Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 135,300,000 135,300,000 135,300,000 224,900,000 213,600,000 206,000,000.0 199,500,000 208,500,000    
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 6,080                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 135,300,000 135,300,000 135,300,000 149,300,000 167,700,000 139,700,000 97,100,000 40,000,000.0    
Accident Year 2017 | Financial and Professional Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 134,400,000 134,400,000 134,400,000 206,800,000 185,200,000 184,600,000 179,700,000 203,700,000    
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 1,758                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 134,400,000 134,400,000 134,400,000 133,500,000 115,800,000 82,700,000 50,900,000 27,000,000.0    
Accident Year 2017 | Property Catastrophe and Other Property Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 431,800,000 431,800,000 431,800,000 568,900,000 496,900,000 508,100,000 524,900,000 544,800,000    
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 1,940                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 431,800,000 431,800,000 431,800,000 407,300,000 434,100,000 410,700,000 353,300,000 122,600,000    
Accident Year 2017 | Casualty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 109,600,000 109,600,000 109,600,000 257,700,000 246,300,000 246,900,000 236,400,000 238,400,000    
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 2,346                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 109,600,000 109,600,000 109,600,000 109,500,000 96,000,000.0 58,100,000 29,900,000 8,700,000    
Accident Year 2017 | Specialty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 303,300,000 303,300,000 303,300,000 352,600,000 359,100,000 370,000,000.0 385,900,000 372,600,000    
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 1,337                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 303,300,000 303,300,000 303,300,000 303,300,000 302,400,000 267,800,000 236,600,000 $ 93,800,000    
Accident Year 2018 | Property Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 184,400,000 184,400,000 184,400,000 194,700,000 184,800,000 200,800,000 198,700,000      
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 8,202                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 184,400,000 184,400,000 184,400,000 172,100,000 178,400,000 156,100,000 60,900,000      
Accident Year 2018 | Casualty Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 42,600,000 42,600,000 42,600,000 162,600,000 133,300,000 123,300,000 120,400,000      
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 5,500                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 42,600,000 42,600,000 42,600,000 57,300,000 42,100,000 27,500,000 3,100,000      
Accident Year 2018 | Marine, Aviation and Energy Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 149,900,000 149,900,000 149,900,000 234,500,000 207,500,000 206,400,000 169,300,000      
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 5,201                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 149,900,000 149,900,000 149,900,000 150,500,000 132,500,000 104,200,000 26,600,000      
Accident Year 2018 | Financial and Professional Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 109,700,000 109,700,000 109,700,000 159,700,000 150,700,000 169,300,000 153,600,000      
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 4,645                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 109,700,000 109,700,000 109,700,000 109,800,000 97,600,000 72,300,000 18,300,000      
Accident Year 2018 | Property Catastrophe and Other Property Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 283,000,000.0 283,000,000.0 283,000,000.0 511,700,000 320,500,000 329,700,000 303,700,000      
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 1,804                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 283,000,000.0 283,000,000.0 283,000,000.0 258,000,000.0 265,600,000 266,800,000 121,400,000      
Accident Year 2018 | Casualty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 91,700,000 91,700,000 91,700,000 251,200,000 260,600,000 252,600,000 223,700,000      
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 2,262                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 91,700,000 91,700,000 91,700,000 91,800,000 72,600,000 33,000,000.0 7,000,000.0      
Accident Year 2018 | Specialty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 322,300,000 322,300,000 322,300,000 412,000,000.0 387,400,000 388,900,000 389,300,000      
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 1,428                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 322,300,000 322,300,000 322,300,000 322,100,000 310,600,000 277,900,000 $ 26,500,000      
Accident Year 2019 | Property Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 189,000,000.0 189,000,000.0 189,000,000.0 102,200,000 128,300,000 124,300,000        
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 6,863                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 189,000,000.0 189,000,000.0 189,000,000.0 97,700,000 89,900,000 48,500,000        
Accident Year 2019 | Casualty Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 48,100,000 48,100,000 48,100,000 152,000,000.0 145,200,000 122,900,000        
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 5,217                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 48,100,000 48,100,000 48,100,000 63,700,000 17,200,000 6,200,000        
Accident Year 2019 | Marine, Aviation and Energy Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 107,000,000.0 107,000,000.0 107,000,000.0 122,700,000 152,200,000 144,600,000        
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 3,699                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 107,000,000.0 107,000,000.0 107,000,000.0 89,300,000 72,100,000 33,300,000        
Accident Year 2019 | Financial and Professional Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 130,800,000 130,800,000 130,800,000 236,300,000 258,100,000 245,900,000        
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 23,819                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 130,800,000 130,800,000 130,800,000 120,400,000 86,200,000 26,800,000        
Accident Year 2019 | Property Catastrophe and Other Property Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 157,500,000 157,500,000 157,500,000 316,300,000 225,000,000.0 209,900,000        
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 1,399                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 157,500,000 157,500,000 157,500,000 158,800,000 134,100,000 28,000,000.0        
Accident Year 2019 | Casualty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 52,200,000 52,200,000 52,200,000 241,500,000 250,800,000 230,100,000        
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 1,939                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 52,200,000 52,200,000 52,200,000 52,000,000.0 36,000,000.0 9,100,000        
Accident Year 2019 | Specialty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 398,600,000 398,600,000 398,600,000 396,900,000 491,700,000 468,600,000        
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 1,557                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 398,600,000 398,600,000 398,600,000 396,900,000 379,000,000.0 $ 272,200,000        
Accident Year 2020 | Property Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 213,000,000.0 212,500,000 207,000,000.0 197,500,000 202,000,000.0          
Incurred but Not Reported (IBNR) Claims Liability, Net $ 23,000,000.0                  
Number of Reported Claims | Integer 7,648                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 178,200,000 169,000,000.0 151,200,000 123,700,000 60,700,000          
Accident Year 2020 | Casualty Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 151,800,000 145,500,000 139,400,000 140,400,000 131,800,000          
Incurred but Not Reported (IBNR) Claims Liability, Net $ 38,100,000                  
Number of Reported Claims | Integer 3,888                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 91,500,000 61,100,000 36,000,000.0 9,300,000 0          
Accident Year 2020 | Marine, Aviation and Energy Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 150,900,000 126,100,000 125,000,000.0 110,700,000 109,600,000          
Incurred but Not Reported (IBNR) Claims Liability, Net $ 33,500,000                  
Number of Reported Claims | Integer 3,866                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 109,200,000 100,900,000 88,500,000 66,400,000 28,500,000          
Accident Year 2020 | Financial and Professional Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 352,200,000 349,800,000 336,200,000 345,500,000 345,200,000          
Incurred but Not Reported (IBNR) Claims Liability, Net $ 73,700,000                  
Number of Reported Claims | Integer 106,042                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 240,000,000.0 225,000,000.0 173,700,000 120,800,000 47,300,000          
Accident Year 2020 | Property Catastrophe and Other Property Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 365,200,000 352,900,000 340,700,000 394,200,000 312,200,000          
Incurred but Not Reported (IBNR) Claims Liability, Net $ 5,200,000                  
Number of Reported Claims | Integer 1,436                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 306,000,000.0 306,000,000.0 232,700,000 162,200,000 41,100,000          
Accident Year 2020 | Casualty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 187,200,000 178,200,000 195,800,000 231,200,000 250,200,000          
Incurred but Not Reported (IBNR) Claims Liability, Net $ 42,400,000                  
Number of Reported Claims | Integer 1,563                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 100,900,000 70,700,000 43,500,000 27,300,000 8,900,000          
Accident Year 2020 | Specialty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 369,500,000 377,100,000 372,900,000 598,700,000 411,200,000          
Incurred but Not Reported (IBNR) Claims Liability, Net $ 15,100,000                  
Number of Reported Claims | Integer 1,519                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 333,000,000.0 310,100,000 289,600,000 269,200,000 $ 212,500,000          
Accident Year 2021 | Property Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 191,100,000 194,600,000 201,000,000.0 206,300,000            
Incurred but Not Reported (IBNR) Claims Liability, Net $ 9,200,000                  
Number of Reported Claims | Integer 6,770                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 162,900,000 150,600,000 119,200,000 58,400,000            
Accident Year 2021 | Casualty Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 194,600,000 196,100,000 186,900,000 171,900,000            
Incurred but Not Reported (IBNR) Claims Liability, Net $ 81,700,000                  
Number of Reported Claims | Integer 3,679                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 91,300,000 53,500,000 23,300,000 3,100,000            
Accident Year 2021 | Marine, Aviation and Energy Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 96,500,000 94,700,000 95,100,000 92,200,000            
Incurred but Not Reported (IBNR) Claims Liability, Net $ 11,900,000                  
Number of Reported Claims | Integer 4,787                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 70,400,000 64,400,000 52,200,000 23,500,000            
Accident Year 2021 | Financial and Professional Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 294,800,000 293,500,000 301,600,000 283,900,000            
Incurred but Not Reported (IBNR) Claims Liability, Net $ 86,200,000                  
Number of Reported Claims | Integer 34,963                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 161,900,000 131,100,000 90,000,000.0 43,100,000            
Accident Year 2021 | Property Catastrophe and Other Property Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 472,500,000 479,800,000 461,200,000 637,700,000            
Incurred but Not Reported (IBNR) Claims Liability, Net $ 4,900,000                  
Number of Reported Claims | Integer 1,518                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 414,800,000 351,500,000 229,200,000 74,600,000            
Accident Year 2021 | Casualty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 199,400,000 203,600,000 213,800,000 203,500,000            
Incurred but Not Reported (IBNR) Claims Liability, Net $ 63,200,000                  
Number of Reported Claims | Integer 1,594                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 96,400,000 63,300,000 36,800,000 7,700,000            
Accident Year 2021 | Specialty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 136,400,000 140,000,000.0 150,300,000 155,200,000            
Incurred but Not Reported (IBNR) Claims Liability, Net $ 24,300,000                  
Number of Reported Claims | Integer 1,385                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 87,900,000 75,200,000 52,800,000 $ 28,000,000.0            
Accident Year 2022 | Property Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 170,900,000 173,200,000 166,000,000.0              
Incurred but Not Reported (IBNR) Claims Liability, Net $ 13,500,000                  
Number of Reported Claims | Integer 5,801                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 143,400,000 113,200,000 41,000,000.0              
Accident Year 2022 | Casualty Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 208,700,000 212,900,000 202,400,000              
Incurred but Not Reported (IBNR) Claims Liability, Net $ 119,400,000                  
Number of Reported Claims | Integer 3,594                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 46,400,000 26,600,000 8,900,000              
Accident Year 2022 | Marine, Aviation and Energy Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 112,700,000 105,300,000 107,200,000              
Incurred but Not Reported (IBNR) Claims Liability, Net $ 25,700,000                  
Number of Reported Claims | Integer 5,410                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 73,500,000 57,500,000 24,900,000              
Accident Year 2022 | Financial and Professional Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 268,600,000 299,800,000 314,100,000              
Incurred but Not Reported (IBNR) Claims Liability, Net $ 121,300,000                  
Number of Reported Claims | Integer 3,356                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 113,400,000 75,100,000 17,700,000              
Accident Year 2022 | Property Catastrophe and Other Property Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 372,100,000 378,900,000 380,500,000              
Incurred but Not Reported (IBNR) Claims Liability, Net $ 43,600,000                  
Number of Reported Claims | Integer 1,475                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 261,300,000 193,600,000 63,300,000              
Accident Year 2022 | Casualty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 246,000,000.0 247,800,000 246,600,000              
Incurred but Not Reported (IBNR) Claims Liability, Net $ 138,500,000                  
Number of Reported Claims | Integer 1,650                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 59,600,000 30,700,000 9,100,000              
Accident Year 2022 | Specialty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 196,200,000 191,600,000 192,600,000              
Incurred but Not Reported (IBNR) Claims Liability, Net $ 98,900,000                  
Number of Reported Claims | Integer 1,458                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 74,300,000 52,600,000 $ 25,300,000              
Accident Year 2023 | Property Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 130,100,000 150,800,000                
Incurred but Not Reported (IBNR) Claims Liability, Net $ 44,300,000                  
Number of Reported Claims 4,056                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 65,200,000 29,800,000                
Accident Year 2023 | Casualty Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 233,800,000 223,100,000                
Incurred but Not Reported (IBNR) Claims Liability, Net $ 156,000,000.0                  
Number of Reported Claims 3,105                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 3,300,000 5,500,000                
Accident Year 2023 | Marine, Aviation and Energy Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 110,300,000 116,600,000                
Incurred but Not Reported (IBNR) Claims Liability, Net $ 20,800,000                  
Number of Reported Claims 3,997                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 73,200,000 27,800,000                
Accident Year 2023 | Financial and Professional Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 347,600,000 338,500,000                
Incurred but Not Reported (IBNR) Claims Liability, Net $ 169,100,000                  
Number of Reported Claims 3,385                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 100,600,000 21,300,000                
Accident Year 2023 | Property Catastrophe and Other Property Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 215,000,000.0 223,600,000                
Incurred but Not Reported (IBNR) Claims Liability, Net $ 40,400,000                  
Number of Reported Claims | Integer 1,104                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 103,800,000 43,900,000                
Accident Year 2023 | Casualty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 267,700,000 268,300,000                
Incurred but Not Reported (IBNR) Claims Liability, Net $ 187,600,000                  
Number of Reported Claims | Integer 1,565                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 35,000,000.0 8,200,000                
Accident Year 2023 | Specialty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 144,300,000 140,700,000                
Incurred but Not Reported (IBNR) Claims Liability, Net $ 73,000,000.0                  
Number of Reported Claims | Integer 1,188                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 48,100,000 $ 22,300,000                
Accident Year 2024 | Property Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 132,600,000                  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 84,200,000                  
Number of Reported Claims 1,879                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 19,100,000                  
Accident Year 2024 | Casualty Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 235,600,000                  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 203,200,000                  
Number of Reported Claims 1,782                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 36,000,000.0                  
Accident Year 2024 | Marine, Aviation and Energy Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 109,600,000                  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 62,300,000                  
Number of Reported Claims 2,358                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 32,400,000                  
Accident Year 2024 | Financial and Professional Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 423,200,000                  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 292,300,000                  
Number of Reported Claims 3,756                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 89,200,000                  
Accident Year 2024 | Property Catastrophe and Other Property Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 254,500,000                  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 135,500,000                  
Number of Reported Claims | Integer 737                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 32,800,000                  
Accident Year 2024 | Casualty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 286,900,000                  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 260,000,000.0                  
Number of Reported Claims | Integer 848                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 9,400,000                  
Accident Year 2024 | Specialty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 153,500,000                  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 125,600,000                  
Number of Reported Claims | Integer 803                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 14,000,000.0