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Reserves for Losses and Adjustment Expenses Short-duration Insurance Contracts, Claims by Accident Year (Details)
Dec. 31, 2024
USD ($)
Dec. 31, 2023
USD ($)
Integer
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]                      
Liability for claims and claim adjustment expenses, net of reinsurance   $ 3,629,600,000 $ 3,151,600,000                
Property Insurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   1,845,700,000                  
Cumulative paid claims and allocated claim adjustment expense, Net   1,574,900,000                  
All outstanding liabilities for 2013 and subsequent years, net of reinsurance   270,800,000                  
All outstanding liabilities before 2013, net of reinsurance   0                  
Liability for claims and claim adjustment expenses, net of reinsurance   270,800,000 259,000,000.0                
Casualty Insurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   1,349,900,000                  
Cumulative paid claims and allocated claim adjustment expense, Net   713,400,000                  
All outstanding liabilities for 2013 and subsequent years, net of reinsurance   636,500,000                  
All outstanding liabilities before 2013, net of reinsurance   0                  
Liability for claims and claim adjustment expenses, net of reinsurance   636,500,000 457,300,000                
Marine, Aviation and Energy Insurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   1,577,100,000                  
Cumulative paid claims and allocated claim adjustment expense, Net   1,382,600,000                  
All outstanding liabilities for 2013 and subsequent years, net of reinsurance   194,500,000                  
All outstanding liabilities before 2013, net of reinsurance   0                  
Liability for claims and claim adjustment expenses, net of reinsurance   194,500,000 163,700,000                
Financial and Professional Insurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   2,051,400,000                  
Cumulative paid claims and allocated claim adjustment expense, Net   1,213,800,000                  
All outstanding liabilities for 2013 and subsequent years, net of reinsurance   837,600,000                  
All outstanding liabilities before 2013, net of reinsurance   0                  
Liability for claims and claim adjustment expenses, net of reinsurance   837,600,000 671,900,000                
Property Catastrophe and Other Property Reinsurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   2,882,400,000                  
Cumulative paid claims and allocated claim adjustment expense, Net   2,320,100,000                  
All outstanding liabilities for 2013 and subsequent years, net of reinsurance   562,300,000                  
All outstanding liabilities before 2013, net of reinsurance   0                  
Liability for claims and claim adjustment expenses, net of reinsurance   562,300,000 677,900,000                
Casualty Reinsurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   1,550,100,000                  
Cumulative paid claims and allocated claim adjustment expense, Net   816,500,000                  
All outstanding liabilities for 2013 and subsequent years, net of reinsurance   733,600,000                  
All outstanding liabilities before 2013, net of reinsurance   0                  
Liability for claims and claim adjustment expenses, net of reinsurance   733,600,000 571,600,000                
Specialty Reinsurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   2,314,000,000                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 1,919,700,000                    
All outstanding liabilities for 2013 and subsequent years, net of reinsurance 394,300,000                    
All outstanding liabilities before 2013, net of reinsurance 0                    
Liability for claims and claim adjustment expenses, net of reinsurance   394,300,000 350,200,000                
Accident Year 2014 | Property Insurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   132,800,000 132,800,000 $ 125,800,000 $ 131,200,000 $ 131,800,000 $ 133,300,000 $ 134,100,000 $ 133,500,000 $ 156,300,000 $ 164,300,000
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 0                  
Number of Reported Claims | Integer   10,037                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 132,800,000 132,800,000 130,900,000 129,800,000 128,500,000 127,100,000 123,100,000 113,500,000 86,000,000.0 40,200,000
Accident Year 2014 | Casualty Insurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   107,000,000.0 107,000,000.0 141,400,000 135,100,000 138,100,000 134,200,000 127,100,000 137,100,000 125,300,000 142,900,000
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 0                  
Number of Reported Claims | Integer   3,962                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 107,000,000.0 107,000,000.0 106,800,000 108,200,000 95,600,000 71,900,000 58,800,000 32,200,000 13,100,000 2,600,000
Accident Year 2014 | Marine, Aviation and Energy Insurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   271,300,000 271,300,000 312,400,000 301,000,000.0 311,700,000 304,800,000 309,200,000 297,400,000 312,300,000 308,800,000
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 0                  
Number of Reported Claims | Integer   4,056                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 271,300,000 271,300,000 269,000,000.0 262,000,000.0 249,800,000 231,600,000 208,900,000 188,300,000 116,300,000 53,300,000
Accident Year 2014 | Financial and Professional Insurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   98,800,000 98,800,000 121,700,000 120,300,000 119,200,000 130,200,000 119,000,000.0 128,600,000 130,100,000 134,300,000
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 0                  
Number of Reported Claims | Integer   794                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 98,800,000 98,800,000 99,300,000 91,700,000 84,900,000 79,200,000 71,800,000 53,200,000 30,500,000 2,900,000
Accident Year 2014 | Property Catastrophe and Other Property Reinsurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   137,100,000 137,100,000 139,700,000 141,100,000 141,100,000 145,800,000 145,700,000 157,200,000 172,800,000 185,800,000
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 0                  
Number of Reported Claims | Integer   901                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 137,100,000 137,100,000 137,400,000 137,500,000 135,900,000 137,300,000 133,800,000 124,500,000 98,700,000 36,800,000
Accident Year 2014 | Casualty Reinsurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   131,000,000.0 131,000,000.0 189,800,000 199,900,000 204,000,000.0 201,000,000.0 207,500,000 214,200,000 205,800,000 203,400,000
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 0                  
Number of Reported Claims | Integer   1,866                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 131,000,000.0 131,000,000.0 131,000,000.0 124,400,000 106,800,000 86,000,000.0 59,900,000 37,600,000 13,700,000 2,500,000
Accident Year 2014 | Specialty Reinsurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   104,000,000.0 104,000,000.0 116,400,000 119,300,000 123,500,000 124,900,000 122,100,000 130,900,000 139,000,000.0 $ 150,700,000
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 0                  
Number of Reported Claims | Integer   618                  
Cumulative paid claims and allocated claim adjustment expense, Net 104,000,000.0 $ 104,000,000.0 104,100,000 103,600,000 101,900,000 99,000,000.0 88,700,000 80,600,000 55,900,000 16,400,000  
Accident Year 2015 | Property Insurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   193,400,000 193,400,000 181,900,000 197,500,000 200,500,000 200,000,000.0 197,700,000 203,000,000.0 237,300,000  
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 0                  
Number of Reported Claims | Integer   11,624                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 193,400,000 193,400,000 193,400,000 192,400,000 193,800,000 177,500,000 168,400,000 141,000,000.0 56,900,000  
Accident Year 2015 | Casualty Insurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   180,500,000 180,500,000 257,900,000 232,300,000 234,000,000.0 201,300,000 183,900,000 221,300,000 201,300,000  
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 0                  
Number of Reported Claims | Integer   4,811                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 180,500,000 180,500,000 180,500,000 166,200,000 137,200,000 91,800,000 56,000,000.0 16,800,000 3,100,000  
Accident Year 2015 | Marine, Aviation and Energy Insurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   267,300,000 267,300,000 318,300,000 311,100,000 308,300,000 284,800,000 280,100,000 297,600,000 295,400,000  
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 0                  
Number of Reported Claims | Integer   4,067                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 267,300,000 267,300,000 268,400,000 256,500,000 220,900,000 193,000,000.0 173,300,000 122,700,000 44,700,000  
Accident Year 2015 | Financial and Professional Insurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   145,900,000 145,900,000 196,700,000 184,600,000 189,800,000 188,700,000 184,600,000 174,800,000 173,300,000  
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 0                  
Number of Reported Claims | Integer   1,083                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 145,900,000 145,900,000 150,900,000 138,400,000 109,500,000 89,100,000 69,900,000 43,300,000 13,700,000  
Accident Year 2015 | Property Catastrophe and Other Property Reinsurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   155,300,000 155,300,000 176,800,000 170,400,000 170,300,000 154,800,000 175,300,000 185,100,000 211,300,000  
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 0                  
Number of Reported Claims | Integer   1,051                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 155,300,000 155,300,000 158,600,000 157,100,000 154,800,000 137,800,000 125,400,000 94,200,000 35,800,000  
Accident Year 2015 | Casualty Reinsurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   116,500,000 116,500,000 195,200,000 205,200,000 209,000,000.0 211,400,000 208,900,000 199,200,000 192,500,000  
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 0                  
Number of Reported Claims | Integer   2,067                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 116,500,000 116,500,000 116,500,000 108,000,000.0 89,000,000.0 65,200,000 38,100,000 17,800,000 3,400,000  
Accident Year 2015 | Specialty Reinsurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   132,100,000 132,100,000 151,100,000 151,400,000 155,900,000 157,300,000 162,700,000 168,000,000.0 $ 164,700,000  
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 0                  
Number of Reported Claims | Integer   774                  
Cumulative paid claims and allocated claim adjustment expense, Net 132,100,000 $ 132,100,000 132,600,000 132,900,000 129,900,000 120,900,000 103,300,000 55,800,000 17,500,000    
Accident Year 2016 | Property Insurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   233,400,000 233,400,000 234,200,000 245,300,000 244,000,000.0 242,600,000 247,700,000 236,800,000    
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 0                  
Number of Reported Claims | Integer   10,844                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 233,400,000 233,400,000 234,900,000 231,100,000 221,900,000 200,200,000 167,800,000 66,500,000    
Accident Year 2016 | Casualty Insurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   124,300,000 124,300,000 243,500,000 198,900,000 187,800,000 181,300,000 186,000,000.0 215,000,000.0    
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 0                  
Number of Reported Claims | Integer   4,815                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 124,300,000 124,300,000 123,300,000 108,400,000 81,500,000 39,600,000 22,500,000 4,100,000    
Accident Year 2016 | Marine, Aviation and Energy Insurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   197,900,000 197,900,000 220,300,000 218,500,000 228,700,000 228,300,000 229,500,000 259,500,000    
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 0                  
Number of Reported Claims | Integer   4,422                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 197,900,000 197,900,000 193,300,000 190,400,000 163,800,000 142,100,000 82,300,000 30,900,000    
Accident Year 2016 | Financial and Professional Insurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   134,300,000 134,300,000 186,100,000 184,600,000 201,200,000 215,400,000 210,900,000 190,100,000    
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 0                  
Number of Reported Claims | Integer   1,241                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 134,300,000 134,300,000 130,100,000 125,500,000 129,600,000 101,500,000 71,000,000.0 15,000,000.0    
Accident Year 2016 | Property Catastrophe and Other Property Reinsurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   228,600,000 228,600,000 230,700,000 239,200,000 243,800,000 265,000,000.0 266,300,000 266,400,000    
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 0                  
Number of Reported Claims | Integer   1,303                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 228,600,000 228,600,000 231,700,000 226,100,000 213,200,000 202,400,000 161,700,000 56,300,000    
Accident Year 2016 | Casualty Reinsurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   137,100,000 137,100,000 251,200,000 260,300,000 252,900,000 243,000,000.0 243,500,000 231,100,000    
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 0                  
Number of Reported Claims | Integer   2,231                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 137,100,000 137,100,000 137,100,000 125,600,000 95,600,000 63,600,000 33,300,000 9,100,000    
Accident Year 2016 | Specialty Reinsurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   188,200,000 188,200,000 211,600,000 224,000,000.0 228,900,000 236,100,000 238,300,000 $ 237,200,000    
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 0                  
Number of Reported Claims | Integer   937                  
Cumulative paid claims and allocated claim adjustment expense, Net 188,200,000 $ 188,200,000 194,300,000 192,900,000 182,700,000 164,700,000 150,300,000 58,400,000      
Accident Year 2017 | Property Insurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   261,600,000 261,600,000 273,000,000.0 251,300,000 250,000,000.0 256,800,000 293,800,000      
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 0                  
Number of Reported Claims | Integer   9,805                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 261,600,000 261,600,000 234,800,000 240,900,000 220,000,000.0 187,300,000 96,000,000.0      
Accident Year 2017 | Casualty Insurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   61,400,000 61,400,000 215,900,000 194,700,000 176,700,000 172,900,000 179,400,000      
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 0                  
Number of Reported Claims | Integer   5,470                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 61,400,000 61,400,000 89,400,000 95,800,000 52,200,000 22,700,000 3,500,000      
Accident Year 2017 | Marine, Aviation and Energy Insurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   141,400,000 141,400,000 225,600,000 214,200,000 206,700,000 200,200,000 209,600,000      
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 0                  
Number of Reported Claims | Integer   6,078                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 141,400,000 141,400,000 149,800,000 168,200,000 140,100,000 97,400,000 40,100,000      
Accident Year 2017 | Financial and Professional Insurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   136,000,000.0 136,000,000.0 209,100,000 187,100,000 186,500,000 181,600,000 205,500,000      
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 0                  
Number of Reported Claims | Integer   1,751                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 136,000,000.0 136,000,000.0 134,800,000 116,800,000 83,200,000 51,200,000 27,100,000      
Accident Year 2017 | Property Catastrophe and Other Property Reinsurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   431,800,000 431,800,000 573,600,000 501,800,000 513,100,000 531,400,000 552,300,000      
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 0                  
Number of Reported Claims | Integer   1,958                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 431,800,000 431,800,000 411,700,000 438,400,000 414,500,000 356,100,000 123,200,000      
Accident Year 2017 | Casualty Reinsurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   110,800,000 110,800,000 261,600,000 250,400,000 251,000,000.0 240,400,000 242,700,000      
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 0                  
Number of Reported Claims | Integer   2,284                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 110,800,000 110,800,000 110,700,000 97,000,000.0 58,800,000 30,400,000 8,800,000      
Accident Year 2017 | Specialty Reinsurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   305,900,000 305,900,000 356,400,000 363,000,000.0 374,100,000 390,400,000 $ 377,400,000      
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 0                  
Number of Reported Claims | Integer   1,336                  
Cumulative paid claims and allocated claim adjustment expense, Net 305,900,000 $ 305,900,000 306,400,000 305,200,000 270,000,000.0 238,200,000 94,500,000        
Accident Year 2018 | Property Insurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   180,200,000 180,200,000 196,300,000 186,800,000 203,000,000.0 200,700,000        
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 0                  
Number of Reported Claims | Integer   8,395                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 180,200,000 180,200,000 174,100,000 180,400,000 158,100,000 61,400,000        
Accident Year 2018 | Casualty Insurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   43,700,000 43,700,000 164,700,000 134,900,000 124,500,000 121,500,000        
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 0                  
Number of Reported Claims | Integer   5,455                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 43,700,000 43,700,000 58,100,000 42,700,000 27,700,000 3,100,000        
Accident Year 2018 | Marine, Aviation and Energy Insurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   151,000,000.0 151,000,000.0 234,800,000 208,300,000 207,400,000 170,400,000        
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 0                  
Number of Reported Claims | Integer   5,183                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 151,000,000.0 151,000,000.0 151,000,000.0 133,000,000.0 104,700,000 26,700,000        
Accident Year 2018 | Financial and Professional Insurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   111,200,000 111,200,000 161,900,000 153,200,000 171,600,000 155,700,000        
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 0                  
Number of Reported Claims | Integer   4,636                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 111,200,000 111,200,000 111,200,000 99,000,000.0 73,400,000 19,100,000        
Accident Year 2018 | Property Catastrophe and Other Property Reinsurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   282,100,000 282,100,000 534,700,000 344,100,000 351,900,000 318,200,000        
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 0                  
Number of Reported Claims | Integer   1,777                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 282,100,000 282,100,000 279,500,000 286,200,000 282,200,000 122,400,000        
Accident Year 2018 | Casualty Reinsurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   92,700,000 92,700,000 254,300,000 264,100,000 256,500,000 227,200,000        
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 0                  
Number of Reported Claims | Integer   2,149                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 92,700,000 92,700,000 92,700,000 73,300,000 33,400,000 7,100,000        
Accident Year 2018 | Specialty Reinsurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   324,300,000 324,300,000 416,200,000 391,700,000 393,300,000 $ 393,800,000        
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 0                  
Number of Reported Claims | Integer   1,418                  
Cumulative paid claims and allocated claim adjustment expense, Net 324,300,000 $ 324,300,000 324,700,000 313,000,000.0 279,600,000 27,100,000          
Accident Year 2019 | Property Insurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   109,000,000.0 109,000,000.0 102,800,000 129,200,000 125,300,000          
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 0                  
Number of Reported Claims | Integer   6,944                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 109,000,000.0 109,000,000.0 98,400,000 90,500,000 48,700,000          
Accident Year 2019 | Casualty Insurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   48,600,000 48,600,000 153,500,000 146,500,000 124,100,000          
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 0                  
Number of Reported Claims | Integer   5,156                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 48,600,000 48,600,000 64,100,000 17,600,000 6,300,000          
Accident Year 2019 | Marine, Aviation and Energy Insurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   102,400,000 102,400,000 123,500,000 153,000,000.0 145,600,000          
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 0                  
Number of Reported Claims | Integer   3,691                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 102,400,000 102,400,000 89,600,000 72,500,000 33,500,000          
Accident Year 2019 | Financial and Professional Insurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   132,700,000 132,700,000 238,900,000 261,100,000 248,200,000          
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 0                  
Number of Reported Claims | Integer   23,826                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 132,700,000 132,700,000 121,100,000 86,600,000 27,100,000          
Accident Year 2019 | Property Catastrophe and Other Property Reinsurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   162,700,000 162,700,000 332,400,000 241,500,000 228,000,000.0          
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 0                  
Number of Reported Claims | Integer   1,401                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 162,700,000 162,700,000 167,500,000 140,800,000 28,100,000          
Accident Year 2019 | Casualty Reinsurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   52,900,000 52,900,000 244,300,000 254,000,000.0 233,500,000          
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 0                  
Number of Reported Claims | Integer   1,790                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 52,900,000 52,900,000 52,500,000 36,400,000 9,200,000          
Accident Year 2019 | Specialty Reinsurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   401,100,000 401,100,000 398,700,000 495,900,000 $ 472,600,000          
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 0                  
Number of Reported Claims | Integer   1,543                  
Cumulative paid claims and allocated claim adjustment expense, Net 401,100,000 $ 401,100,000 399,200,000 381,000,000.0 273,200,000            
Accident Year 2020 | Property Insurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   213,500,000 208,000,000.0 198,300,000 203,200,000            
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 25,900,000                  
Number of Reported Claims | Integer   7,689                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 169,600,000 151,800,000 124,100,000 61,000,000.0            
Accident Year 2020 | Casualty Insurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   146,600,000 140,400,000 141,600,000 132,900,000            
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 50,200,000                  
Number of Reported Claims | Integer   3,753                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 61,600,000 36,300,000 9,300,000 0            
Accident Year 2020 | Marine, Aviation and Energy Insurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   126,600,000 125,700,000 111,200,000 110,200,000            
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 9,600,000                  
Number of Reported Claims | Integer   3,847                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 101,200,000 88,700,000 66,500,000 28,500,000            
Accident Year 2020 | Financial and Professional Insurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   352,300,000 338,200,000 347,900,000 348,100,000            
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 87,400,000                  
Number of Reported Claims | Integer   106,054                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 226,200,000 174,700,000 121,200,000 47,600,000            
Accident Year 2020 | Property Catastrophe and Other Property Reinsurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   362,600,000 350,700,000 403,700,000 317,600,000            
Incurred but Not Reported (IBNR) Claims Liability, Net   $ (5,500,000)                  
Number of Reported Claims | Integer   1,418                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 312,300,000 237,300,000 165,700,000 41,900,000            
Accident Year 2020 | Casualty Reinsurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   181,200,000 198,800,000 234,700,000 253,600,000            
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 62,000,000.0                  
Number of Reported Claims | Integer   1,389                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 71,900,000 44,300,000 27,800,000 9,100,000            
Accident Year 2020 | Specialty Reinsurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   379,500,000 375,400,000 601,300,000 $ 414,100,000            
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 36,700,000                  
Number of Reported Claims | Integer   1,494                  
Cumulative paid claims and allocated claim adjustment expense, Net 311,600,000 $ 290,900,000 270,100,000 213,000,000.0              
Accident Year 2021 | Property Insurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   195,500,000 202,000,000.0 207,400,000              
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 16,100,000                  
Number of Reported Claims | Integer   6,810                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 151,200,000 119,700,000 58,700,000              
Accident Year 2021 | Casualty Insurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   197,800,000 188,600,000 173,500,000              
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 99,400,000                  
Number of Reported Claims | Integer   3,343                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 53,800,000 23,500,000 3,100,000              
Accident Year 2021 | Marine, Aviation and Energy Insurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   95,700,000 96,200,000 93,100,000              
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 10,100,000                  
Number of Reported Claims | Integer   4,787                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 64,700,000 52,300,000 23,500,000              
Accident Year 2021 | Financial and Professional Insurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   296,200,000 304,800,000 286,200,000              
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 128,400,000                  
Number of Reported Claims | Integer   34,929                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 131,700,000 90,400,000 43,200,000              
Accident Year 2021 | Property Catastrophe and Other Property Reinsurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   498,900,000 479,300,000 652,900,000              
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 30,700,000                  
Number of Reported Claims | Integer   1,498                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 364,300,000 235,700,000 75,000,000.0              
Accident Year 2021 | Casualty Reinsurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   205,700,000 216,300,000 206,400,000              
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 93,300,000                  
Number of Reported Claims | Integer   1,325                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 64,000,000.0 37,300,000 7,800,000              
Accident Year 2021 | Specialty Reinsurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   142,100,000 152,500,000 $ 157,400,000              
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 41,100,000                  
Number of Reported Claims | Integer   1,356                  
Cumulative paid claims and allocated claim adjustment expense, Net 76,100,000 $ 53,400,000 28,300,000                
Accident Year 2022 | Property Insurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   174,200,000 167,100,000                
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 22,300,000                  
Number of Reported Claims   5,702                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 113,700,000 41,100,000                
Accident Year 2022 | Casualty Insurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   215,100,000 204,700,000                
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 148,300,000                  
Number of Reported Claims   3,050                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 5,600,000 8,900,000                
Accident Year 2022 | Marine, Aviation and Energy Insurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   106,100,000 108,000,000.0                
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 31,200,000                  
Number of Reported Claims   5,428                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 57,600,000 24,900,000                
Accident Year 2022 | Financial and Professional Insurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   302,400,000 317,000,000.0                
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 184,600,000                  
Number of Reported Claims   3,422                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 75,500,000 17,800,000                
Accident Year 2022 | Property Catastrophe and Other Property Reinsurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   392,500,000 393,700,000                
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 48,900,000                  
Number of Reported Claims | Integer   1,404                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 200,800,000 65,200,000                
Accident Year 2022 | Casualty Reinsurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   250,600,000 249,500,000                
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 178,300,000                  
Number of Reported Claims | Integer   1,256                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 31,100,000 9,400,000                
Accident Year 2022 | Specialty Reinsurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   194,300,000 $ 194,700,000                
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 108,900,000                  
Number of Reported Claims | Integer   1,319                  
Cumulative paid claims and allocated claim adjustment expense, Net 53,800,000 $ 26,000,000.0                  
Short-Duration Insurance Contract, Accident Year 2023 | Property Insurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   152,100,000                  
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 81,700,000                  
Number of Reported Claims   3,297                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 30,000,000.0                  
Short-Duration Insurance Contract, Accident Year 2023 | Casualty Insurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   224,900,000                  
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 173,000,000.0                  
Number of Reported Claims   2,465                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 26,900,000                  
Short-Duration Insurance Contract, Accident Year 2023 | Marine, Aviation and Energy Insurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   117,400,000                  
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 72,600,000                  
Number of Reported Claims   2,703                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 27,800,000                  
Short-Duration Insurance Contract, Accident Year 2023 | Financial and Professional Insurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   341,600,000                  
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 277,000,000.0                  
Number of Reported Claims   3,137                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 21,500,000                  
Short-Duration Insurance Contract, Accident Year 2023 | Property Catastrophe and Other Property Reinsurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   230,800,000                  
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 117,700,000                  
Number of Reported Claims | Integer   753                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 45,100,000                  
Short-Duration Insurance Contract, Accident Year 2023 | Casualty Reinsurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   271,600,000                  
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 244,900,000                  
Number of Reported Claims | Integer   779                  
Cumulative paid claims and allocated claim adjustment expense, Net   $ 8,500,000                  
Short-Duration Insurance Contract, Accident Year 2023 | Specialty Reinsurance                      
Claims Development [Line Items]                      
Incurred Claims and Allocated Claim Adjustment Expense, Net   142,500,000                  
Incurred but Not Reported (IBNR) Claims Liability, Net   $ 100,400,000                  
Number of Reported Claims | Integer   952                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 22,600,000