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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 ($)
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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 |  |  |  |  |  |  |  |  |  |

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