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Reserves for Losses and Adjustment Expenses Short-duration Insurance Contracts, Claims by Accident Year (Details)
Dec. 31, 2022
USD ($)
Integer
Dec. 31, 2021
USD ($)
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 ($)
Dec. 31, 2013
USD ($)
Claims Development [Line Items]                    
Liability for claims and claim adjustment expenses, net of reinsurance $ 3,151,600,000 $ 4,484,000,000                
Property Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 1,772,500,000                  
Cumulative paid claims and allocated claim adjustment expense, Net 1,513,500,000                  
All outstanding liabilities for 2013 and subsequent years, net of reinsurance 259,000,000.0                  
All outstanding liabilities before 2013, net of reinsurance 0                  
Liability for claims and claim adjustment expenses, net of reinsurance 259,000,000.0 289,300,000                
Casualty Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 1,417,100,000                  
Cumulative paid claims and allocated claim adjustment expense, Net 959,800,000                  
All outstanding liabilities for 2013 and subsequent years, net of reinsurance 457,300,000                  
All outstanding liabilities before 2013, net of reinsurance 0                  
Liability for claims and claim adjustment expenses, net of reinsurance 457,300,000 725,600,000                
Marine, Aviation and Energy Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 1,929,200,000                  
Cumulative paid claims and allocated claim adjustment expense, Net 1,765,500,000                  
All outstanding liabilities for 2013 and subsequent years, net of reinsurance 163,700,000                  
All outstanding liabilities before 2013, net of reinsurance 0                  
Liability for claims and claim adjustment expenses, net of reinsurance 163,700,000 427,900,000                
Financial and Professional Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 1,970,000,000                  
Cumulative paid claims and allocated claim adjustment expense, Net 1,298,100,000                  
All outstanding liabilities for 2013 and subsequent years, net of reinsurance 671,900,000                  
All outstanding liabilities before 2013, net of reinsurance 0                  
Liability for claims and claim adjustment expenses, net of reinsurance 671,900,000 808,700,000                
Property Catastrophe and Other Property Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 2,854,500,000                  
Cumulative paid claims and allocated claim adjustment expense, Net 2,176,600,000                  
All outstanding liabilities for 2013 and subsequent years, net of reinsurance 677,900,000                  
All outstanding liabilities before 2013, net of reinsurance 0                  
Liability for claims and claim adjustment expenses, net of reinsurance 677,900,000 784,900,000                
Casualty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 1,628,700,000                  
Cumulative paid claims and allocated claim adjustment expense, Net 1,057,100,000                  
All outstanding liabilities for 2013 and subsequent years, net of reinsurance 571,600,000                  
All outstanding liabilities before 2013, net of reinsurance 0                  
Liability for claims and claim adjustment expenses, net of reinsurance 571,600,000 975,300,000                
Specialty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 2,344,800,000                  
Cumulative paid claims and allocated claim adjustment expense, Net 1,994,600,000                  
All outstanding liabilities for 2013 and subsequent years, net of reinsurance 350,200,000                  
All outstanding liabilities before 2013, net of reinsurance 0                  
Liability for claims and claim adjustment expenses, net of reinsurance 350,200,000 472,300,000                
Accident Year 2013 | Property Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 106,100,000 102,100,000 $ 109,500,000 $ 109,300,000 $ 109,000,000.0 $ 110,900,000 $ 109,900,000 $ 114,200,000 $ 114,400,000 $ 126,700,000
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 0                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 106,100,000 106,800,000 107,900,000 107,600,000 105,800,000 103,000,000.0 98,600,000 86,800,000 73,700,000 37,500,000
Accident Year 2013 | Casualty Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 86,700,000 107,800,000 100,000,000.0 101,400,000 101,100,000 99,800,000 116,800,000 111,600,000 112,800,000 128,600,000
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 3,392                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 86,700,000 84,000,000.0 83,900,000 82,000,000.0 77,900,000 65,900,000 51,200,000 38,200,000 25,100,000 2,100,000
Accident Year 2013 | Marine, Aviation and Energy Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 319,700,000 346,500,000 338,700,000 343,600,000 345,000,000.0 331,500,000 324,300,000 340,700,000 332,100,000 319,400,000
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 4,179                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 319,700,000 309,000,000.0 309,300,000 299,100,000 283,100,000 263,500,000 234,100,000 203,900,000 130,900,000 41,200,000
Accident Year 2013 | Financial and Professional Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 82,900,000 101,000,000.0 95,100,000 88,800,000 89,900,000 98,600,000 99,600,000 103,100,000 98,700,000 104,300,000
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 606                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 82,900,000 79,900,000 76,400,000 73,200,000 71,100,000 62,800,000 64,600,000 30,600,000 20,600,000 7,900,000
Accident Year 2013 | Property Catastrophe and Other Property Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 156,300,000 159,500,000 162,900,000 165,800,000 168,500,000 171,700,000 173,100,000 184,100,000 193,700,000 212,300,000
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 832                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 156,300,000 156,000,000.0 156,000,000.0 161,600,000 160,100,000 158,600,000 154,100,000 142,600,000 95,700,000 33,900,000
Accident Year 2013 | Casualty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 151,900,000 191,600,000 197,900,000 200,300,000 197,700,000 202,300,000 219,100,000 221,600,000 226,700,000 211,700,000
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 1,700                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 151,900,000 144,200,000 137,900,000 126,000,000.0 113,500,000 91,800,000 64,400,000 42,300,000 15,700,000 3,400,000
Accident Year 2013 | Specialty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 98,100,000 104,800,000 108,300,000 111,600,000 111,900,000 115,700,000 116,300,000 127,700,000 135,700,000 140,500,000
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 579                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 98,100,000 98,100,000 99,000,000.0 99,300,000 98,400,000 98,400,000 91,400,000 84,400,000 68,700,000 $ 24,300,000
Accident Year 2014 | Property Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 131,100,000 125,200,000 130,100,000 130,700,000 132,200,000 133,000,000.0 132,400,000 154,900,000 162,600,000  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 9,949                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 131,100,000 129,900,000 128,700,000 127,300,000 126,000,000.0 122,000,000.0 112,500,000 85,200,000 39,900,000  
Accident Year 2014 | Casualty Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 107,400,000 137,900,000 131,800,000 134,900,000 131,000,000.0 124,000,000.0 134,200,000 122,600,000 139,900,000  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 3,914                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 107,400,000 104,000,000.0 105,400,000 93,100,000 70,100,000 57,300,000 31,500,000 12,600,000 2,500,000  
Accident Year 2014 | Marine, Aviation and Energy Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 279,100,000 317,600,000 300,700,000 311,400,000 304,500,000 308,900,000 297,200,000 312,200,000 308,600,000  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 4,058                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 279,100,000 274,200,000 261,700,000 249,500,000 231,300,000 208,600,000 188,100,000 116,000,000.0 53,200,000  
Accident Year 2014 | Financial and Professional Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 118,900,000 124,300,000 118,800,000 118,000,000.0 128,900,000 117,700,000 127,400,000 128,800,000 133,200,000  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 794                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 118,900,000 102,300,000 90,900,000 84,000,000.0 78,500,000 71,100,000 52,700,000 30,200,000 2,800,000  
Accident Year 2014 | Property Catastrophe and Other Property Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 138,100,000 140,100,000 141,200,000 141,100,000 145,900,000 145,700,000 157,100,000 172,600,000 185,800,000  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 900                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 138,100,000 137,800,000 137,600,000 136,000,000.0 137,400,000 133,700,000 124,400,000 98,500,000 36,900,000  
Accident Year 2014 | Casualty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 144,700,000 191,100,000 198,000,000.0 202,100,000 199,000,000.0 205,500,000 212,300,000 204,000,000.0 201,800,000  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 1,843                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 144,700,000 133,000,000.0 123,400,000 105,900,000 85,400,000 59,600,000 37,400,000 13,600,000 2,500,000  
Accident Year 2014 | Specialty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 105,400,000 116,600,000 118,000,000.0 122,200,000 123,600,000 120,800,000 129,500,000 137,500,000 149,500,000  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 622                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 105,400,000 104,400,000 102,500,000 100,900,000 98,000,000.0 87,800,000 79,800,000 55,200,000 $ 16,200,000  
Accident Year 2015 | Property Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 189,200,000 179,600,000 193,900,000 196,900,000 196,300,000 193,900,000 199,000,000.0 233,300,000    
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 11,549                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 189,200,000 190,400,000 188,900,000 190,100,000 174,100,000 165,100,000 138,300,000 56,300,000    
Accident Year 2015 | Casualty Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 197,200,000 250,400,000 228,300,000 230,100,000 197,500,000 180,500,000 217,900,000 197,900,000    
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 4,775                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 197,200,000 174,500,000 163,600,000 135,000,000.0 90,200,000 55,100,000 16,400,000 3,000,000.0    
Accident Year 2015 | Marine, Aviation and Energy Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 302,700,000 327,300,000 309,900,000 307,200,000 283,700,000 279,300,000 297,000,000.0 295,300,000    
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 4,076                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 302,700,000 277,500,000 255,500,000 220,000,000.0 192,100,000 172,700,000 122,500,000 44,600,000    
Accident Year 2015 | Financial and Professional Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 163,900,000 189,900,000 183,000,000.0 188,000,000.0 187,100,000 183,100,000 173,400,000 171,900,000    
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 1,085                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 163,900,000 144,100,000 137,600,000 108,800,000 88,600,000 69,500,000 43,100,000 13,500,000    
Accident Year 2015 | Property Catastrophe and Other Property Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 160,600,000 177,200,000 168,900,000 168,900,000 153,500,000 173,800,000 183,700,000 210,000,000.0    
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 1,048                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 160,600,000 159,500,000 155,900,000 153,600,000 136,700,000 124,200,000 93,600,000 35,600,000    
Accident Year 2015 | Casualty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 135,100,000 198,800,000 203,700,000 207,500,000 209,800,000 207,300,000 197,600,000 191,100,000    
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 2,038                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 135,100,000 121,400,000 107,300,000 88,400,000 64,700,000 37,800,000 17,500,000 3,300,000    
Accident Year 2015 | Specialty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 132,500,000 150,000,000.0 150,000,000.0 154,500,000 155,900,000 161,100,000 166,400,000 163,200,000    
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 774                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 132,500,000 131,700,000 131,700,000 128,700,000 119,900,000 102,500,000 55,200,000 $ 17,300,000    
Accident Year 2016 | Property Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 244,800,000 233,400,000 242,500,000 241,300,000 240,000,000.0 245,000,000.0 233,900,000      
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 10,773                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 244,800,000 233,700,000 228,600,000 219,200,000 198,100,000 166,100,000 65,700,000      
Accident Year 2016 | Casualty Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 163,300,000 247,400,000 195,100,000 184,200,000 177,600,000 182,500,000 211,500,000      
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 4,818                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 163,300,000 128,800,000 106,100,000 79,700,000 38,600,000 22,000,000.0 4,000,000.0      
Accident Year 2016 | Marine, Aviation and Energy Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 199,200,000 224,300,000 217,700,000 227,900,000 227,500,000 229,000,000.0 258,800,000      
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 4,431                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 199,200,000 197,300,000 189,500,000 163,100,000 141,300,000 81,900,000 30,800,000      
Accident Year 2016 | Financial and Professional Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 163,200,000 189,000,000.0 182,900,000 199,700,000 213,700,000 209,300,000 188,400,000      
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 $ 163,200,000 133,800,000 124,600,000 128,700,000 100,700,000 70,700,000 14,800,000      
Accident Year 2016 | Property Catastrophe and Other Property Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 229,800,000 230,100,000 237,700,000 242,100,000 263,200,000 264,600,000 264,200,000      
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 1,307                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 229,800,000 230,700,000 224,400,000 211,600,000 200,900,000 160,500,000 55,800,000      
Accident Year 2016 | Casualty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 174,600,000 258,500,000 258,500,000 251,000,000.0 241,000,000.0 241,400,000 229,300,000      
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 2,153                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 174,600,000 145,300,000 124,600,000 94,800,000 62,900,000 32,900,000 8,900,000      
Accident Year 2016 | Specialty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 197,600,000 218,800,000 222,600,000 227,600,000 234,600,000 236,600,000 235,800,000      
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 938                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 197,600,000 201,800,000 191,800,000 181,700,000 163,900,000 149,500,000 $ 58,200,000      
Accident Year 2017 | Property Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 247,700,000 280,900,000 248,100,000 246,800,000 253,700,000 290,400,000        
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 9,717                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 247,700,000 242,100,000 237,900,000 217,200,000 185,000,000.0 95,000,000.0        
Accident Year 2017 | Casualty Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 154,400,000 234,600,000 190,500,000 173,400,000 169,900,000 176,300,000        
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 5,495                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 154,400,000 110,300,000 93,400,000 51,200,000 22,300,000 3,400,000        
Accident Year 2017 | Marine, Aviation and Energy Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 205,300,000 231,500,000 213,700,000 206,200,000 199,900,000 209,300,000        
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 6,059                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 205,300,000 155,700,000 167,600,000 139,600,000 97,000,000.0 40,000,000.0        
Accident Year 2017 | Financial and Professional Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 166,700,000 208,800,000 185,200,000 184,500,000 179,600,000 203,900,000        
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 1,737                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 166,700,000 135,600,000 116,200,000 83,000,000.0 51,000,000.0 27,100,000        
Accident Year 2017 | Property Catastrophe and Other Property Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 447,700,000 595,500,000 500,400,000 511,700,000 529,400,000 550,700,000        
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 1,953                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 447,700,000 433,700,000 437,100,000 413,500,000 355,400,000 123,000,000.0        
Accident Year 2017 | Casualty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 147,700,000 265,500,000 248,900,000 249,700,000 239,100,000 241,000,000.0        
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 2,166                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 147,700,000 115,700,000 96,500,000 58,500,000 30,200,000 8,700,000        
Accident Year 2017 | Specialty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 325,900,000 364,200,000 361,100,000 372,100,000 388,400,000 375,400,000        
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 $ 325,900,000 314,400,000 303,800,000 268,900,000 237,400,000 $ 94,100,000        
Accident Year 2018 | Property Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 177,300,000 196,200,000 182,200,000 198,100,000 196,300,000          
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 8,308                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 177,300,000 173,000,000.0 176,100,000 153,600,000 60,300,000          
Accident Year 2018 | Casualty Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 94,400,000 169,700,000 132,200,000 122,500,000 119,400,000          
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 $ 94,400,000 65,500,000 41,400,000 27,200,000 3,000,000.0          
Accident Year 2018 | Marine, Aviation and Energy Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 169,700,000 238,800,000 207,900,000 207,100,000 170,100,000          
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 5,153                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 169,700,000 154,700,000 132,600,000 104,300,000 26,700,000          
Accident Year 2018 | Financial and Professional Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 125,400,000 163,200,000 151,200,000 169,400,000 153,700,000          
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 4,643                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 125,400,000 113,300,000 97,800,000 71,600,000 17,400,000          
Accident Year 2018 | Property Catastrophe and Other Property Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 304,500,000 571,000,000.0 356,000,000.0 362,800,000 323,500,000          
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 1,780                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 304,500,000 315,300,000 296,800,000 290,100,000 122,100,000          
Accident Year 2018 | Casualty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 130,400,000 264,300,000 262,500,000 255,000,000.0 226,000,000.0          
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 2,002                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 130,400,000 103,800,000 73,000,000.0 33,200,000 7,100,000          
Accident Year 2018 | Specialty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 342,100,000 419,500,000 390,300,000 391,900,000 392,700,000          
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 1,403                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 342,100,000 328,500,000 312,200,000 279,000,000.0 $ 26,700,000          
Accident Year 2019 | Property Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 107,800,000 104,100,000 126,500,000 122,500,000            
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 6,852                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 107,800,000 99,300,000 88,600,000 48,000,000.0            
Accident Year 2019 | Casualty Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 88,300,000 155,300,000 144,400,000 122,000,000.0            
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 5,198                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 88,300,000 68,000,000.0 16,700,000 6,200,000            
Accident Year 2019 | Marine, Aviation and Energy Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 124,000,000.0 151,100,000 152,800,000 145,500,000            
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 3,662                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 124,000,000.0 117,500,000 72,400,000 33,400,000            
Accident Year 2019 | Financial and Professional Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 195,200,000 253,900,000 259,600,000 246,800,000            
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 23,831                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 195,200,000 136,800,000 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 203,600,000 358,700,000 249,600,000 237,300,000            
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 $ 203,600,000 191,400,000 143,900,000 28,100,000            
Accident Year 2019 | Casualty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 82,100,000 252,000,000.0 253,100,000 232,500,000            
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 1,567                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 82,100,000 60,900,000 36,200,000 9,100,000            
Accident Year 2019 | Specialty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 423,600,000 397,400,000 493,400,000 470,300,000            
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 1,537                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 423,600,000 398,800,000 379,600,000 $ 272,500,000            
Accident Year 2020 | Property Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 205,100,000 195,900,000 199,900,000              
Incurred but Not Reported (IBNR) Claims Liability, Net $ 24,400,000                  
Number of Reported Claims | Integer 7,542                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 150,100,000 122,900,000 60,100,000              
Accident Year 2020 | Casualty Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 138,700,000 139,300,000 131,100,000              
Incurred but Not Reported (IBNR) Claims Liability, Net $ 72,100,000                  
Number of Reported Claims | Integer 3,755                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 36,100,000 9,200,000 0              
Accident Year 2020 | Marine, Aviation and Energy Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 125,700,000 111,200,000 110,200,000              
Incurred but Not Reported (IBNR) Claims Liability, Net $ 17,500,000                  
Number of Reported Claims | Integer 3,856                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 88,600,000 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 336,200,000 346,200,000 346,300,000              
Incurred but Not Reported (IBNR) Claims Liability, Net $ 118,000,000.0                  
Number of Reported Claims | Integer 105,947                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 174,200,000 120,800,000 47,200,000              
Accident Year 2020 | Property Catastrophe and Other Property Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 348,900,000 402,100,000 317,500,000              
Incurred but Not Reported (IBNR) Claims Liability, Net $ 81,000,000.0                  
Number of Reported Claims | Integer 1,379                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 237,300,000 165,500,000 41,800,000              
Accident Year 2020 | Casualty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 197,900,000 233,600,000 252,600,000              
Incurred but Not Reported (IBNR) Claims Liability, Net $ 123,200,000                  
Number of Reported Claims | Integer 1,088                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 44,100,000 27,600,000 9,100,000              
Accident Year 2020 | Specialty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 374,100,000 600,000,000.0 412,800,000              
Incurred but Not Reported (IBNR) Claims Liability, Net $ 53,100,000                  
Number of Reported Claims | Integer 1,470                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 290,400,000 269,600,000 $ 212,800,000              
Accident Year 2021 | Property Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 199,500,000 204,600,000                
Incurred but Not Reported (IBNR) Claims Liability, Net $ 38,200,000                  
Number of Reported Claims 6,583                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 118,600,000 58,200,000                
Accident Year 2021 | Casualty Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 185,600,000 170,500,000                
Incurred but Not Reported (IBNR) Claims Liability, Net $ 119,700,000                  
Number of Reported Claims 3,356                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 8,800,000 3,000,000.00                
Accident Year 2021 | Marine, Aviation and Energy Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 96,100,000 93,000,000.0                
Incurred but Not Reported (IBNR) Claims Liability, Net $ 18,300,000                  
Number of Reported Claims 4,863                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 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 302,800,000 284,500,000                
Incurred but Not Reported (IBNR) Claims Liability, Net $ 170,400,000                  
Number of Reported Claims 34,942                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 90,200,000 42,800,000                
Accident Year 2021 | Property Catastrophe and Other Property Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 472,700,000 647,300,000                
Incurred but Not Reported (IBNR) Claims Liability, Net $ 196,900,000                  
Number of Reported Claims | Integer 1,411                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 233,200,000 75,000,000.0                
Accident Year 2021 | Casualty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 215,600,000 205,700,000                
Incurred but Not Reported (IBNR) Claims Liability, Net $ 135,900,000                  
Number of Reported Claims | Integer 815                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 37,100,000 7,800,000                
Accident Year 2021 | Specialty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 151,600,000 156,600,000                
Incurred but Not Reported (IBNR) Claims Liability, Net $ 55,800,000                  
Number of Reported Claims | Integer 1,276                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 53,200,000 $ 28,200,000                
Accident Year 2022 | Property Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 163,900,000                  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 62,900,000                  
Number of Reported Claims 4,171                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 40,800,000                  
Accident Year 2022 | Casualty Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 201,100,000                  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 132,100,000                  
Number of Reported Claims 2,709                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 23,200,000                  
Accident Year 2022 | Marine, Aviation and Energy Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 107,700,000                  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 58,800,000                  
Number of Reported Claims 3,810                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 24,900,000                  
Accident Year 2022 | Financial and Professional Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 314,800,000                  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 265,200,000                  
Number of Reported Claims 3,487                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 17,500,000                  
Accident Year 2022 | Property Catastrophe and Other Property Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 392,300,000                  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 266,700,000                  
Number of Reported Claims | Integer 950                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 65,500,000                  
Accident Year 2022 | Casualty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 248,700,000                  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 179,100,000                  
Number of Reported Claims | Integer 478                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 9,400,000                  
Accident Year 2022 | Specialty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 193,900,000                  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 107,900,000                  
Number of Reported Claims | Integer 969                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 25,800,000