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Reserves for Losses and Adjustment Expenses Short-duration Insurance Contracts, Claims by Accident Year (Details)
Dec. 31, 2021
USD ($)
Integer
Dec. 31, 2020
USD ($)
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 ($)
Dec. 31, 2012
USD ($)
Claims Development [Line Items]                    
Liability for claims and claim adjustment expenses, net of reinsurance $ 4,484,000,000 $ 4,266,000,000                
Property Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 1,840,300,000                  
Cumulative paid claims and allocated claim adjustment expense, Net 1,553,000,000                  
All outstanding liabilities for 2012 and subsequent years, net of reinsurance 287,300,000                  
All outstanding liabilities before 2012, net of reinsurance 2,000,000.0                  
Liability for claims and claim adjustment expenses, net of reinsurance 289,300,000 226,800,000                
Casualty Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 1,537,500,000                  
Cumulative paid claims and allocated claim adjustment expense, Net 848,900,000                  
All outstanding liabilities for 2012 and subsequent years, net of reinsurance 688,600,000                  
All outstanding liabilities before 2012, net of reinsurance 37,000,000.0                  
Liability for claims and claim adjustment expenses, net of reinsurance 725,600,000 618,700,000                
Marine, Aviation and Energy Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 2,289,200,000                  
Cumulative paid claims and allocated claim adjustment expense, Net 1,871,900,000                  
All outstanding liabilities for 2012 and subsequent years, net of reinsurance 417,300,000                  
All outstanding liabilities before 2012, net of reinsurance 10,600,000                  
Liability for claims and claim adjustment expenses, net of reinsurance 427,900,000 431,400,000                
Financial and Professional Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 1,921,500,000                  
Cumulative paid claims and allocated claim adjustment expense, Net 1,119,300,000                  
All outstanding liabilities for 2012 and subsequent years, net of reinsurance 802,200,000                  
All outstanding liabilities before 2012, net of reinsurance 6,500,000                  
Liability for claims and claim adjustment expenses, net of reinsurance 808,700,000 738,500,000                
Property Catastrophe and Other Property Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 2,951,100,000                  
Cumulative paid claims and allocated claim adjustment expense, Net 2,183,600,000                  
All outstanding liabilities for 2012 and subsequent years, net of reinsurance 767,500,000                  
All outstanding liabilities before 2012, net of reinsurance 17,400,000                  
Liability for claims and claim adjustment expenses, net of reinsurance 784,900,000 631,000,000.0                
Casualty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 1,907,200,000                  
Cumulative paid claims and allocated claim adjustment expense, Net 1,038,200,000                  
All outstanding liabilities for 2012 and subsequent years, net of reinsurance 869,000,000.0                  
All outstanding liabilities before 2012, net of reinsurance 106,300,000                  
Liability for claims and claim adjustment expenses, net of reinsurance 975,300,000 1,084,200,000                
Specialty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 2,507,600,000                  
Cumulative paid claims and allocated claim adjustment expense, Net 2,049,900,000                  
All outstanding liabilities for 2012 and subsequent years, net of reinsurance 457,700,000                  
All outstanding liabilities before 2012, net of reinsurance 14,600,000                  
Liability for claims and claim adjustment expenses, net of reinsurance 472,300,000 535,400,000                
Accident Year 2012 | Property Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 155,600,000 154,400,000 $ 153,800,000 $ 153,700,000 $ 154,800,000 $ 160,600,000 $ 166,200,000 $ 167,300,000 $ 168,600,000 $ 170,600,000
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 6,083                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 155,600,000 154,300,000 154,300,000 154,300,000 155,100,000 157,600,000 153,400,000 139,200,000 129,600,000 41,500,000
Accident Year 2012 | Casualty Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 69,300,000 71,500,000 71,100,000 69,200,000 67,200,000 69,600,000 61,700,000 70,600,000 63,400,000 78,600,000
Incurred but Not Reported (IBNR) Claims Liability, Net $ 3,400,000                  
Number of Reported Claims | Integer 3,130                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 65,000,000.0 59,900,000 55,200,000 50,600,000 49,700,000 41,300,000 30,000,000.0 14,300,000 6,700,000 1,300,000
Accident Year 2012 | Marine, Aviation and Energy Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 308,800,000 306,100,000 311,200,000 316,500,000 328,200,000 332,100,000 346,400,000 325,800,000 306,200,000 268,800,000
Incurred but Not Reported (IBNR) Claims Liability, Net $ (6,100,000)                  
Number of Reported Claims | Integer 3,823                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 284,800,000 276,900,000 274,700,000 273,800,000 250,900,000 240,000,000.0 211,200,000 174,900,000 132,400,000 51,500,000
Accident Year 2012 | Financial and Professional Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 104,500,000 103,400,000 96,500,000 101,700,000 89,600,000 94,000,000.0 97,000,000.0 93,600,000 89,900,000 88,500,000
Incurred but Not Reported (IBNR) Claims Liability, Net $ (4,400,000)                  
Number of Reported Claims | Integer 579                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 92,800,000 91,300,000 85,300,000 80,300,000 70,600,000 65,400,000 59,400,000 50,800,000 39,600,000 22,800,000
Accident Year 2012 | Property Catastrophe and Other Property Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 248,600,000 264,900,000 261,600,000 271,700,000 279,200,000 282,200,000 278,800,000 286,200,000 303,100,000 280,200,000
Incurred but Not Reported (IBNR) Claims Liability, Net $ (3,100,000)                  
Number of Reported Claims | Integer 679                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 248,600,000 247,900,000 242,200,000 232,100,000 227,600,000 216,400,000 208,900,000 188,800,000 135,700,000 35,600,000
Accident Year 2012 | Casualty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 196,200,000 240,000,000.0 245,100,000 242,600,000 233,700,000 232,800,000 236,000,000.0 244,600,000 233,300,000 235,100,000
Incurred but Not Reported (IBNR) Claims Liability, Net $ (27,100,000)                  
Number of Reported Claims | Integer 1,813                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 165,400,000 162,200,000 144,400,000 134,700,000 117,800,000 96,600,000 65,500,000 42,100,000 17,800,000 2,200,000
Accident Year 2012 | Specialty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 156,300,000 165,600,000 166,600,000 169,100,000 172,400,000 171,700,000 173,300,000 188,800,000 199,500,000 176,200,000
Incurred but Not Reported (IBNR) Claims Liability, Net $ (3,900,000)                  
Number of Reported Claims | Integer 641                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 156,100,000 153,400,000 152,900,000 150,100,000 148,800,000 143,500,000 138,200,000 128,100,000 93,000,000.0 $ 24,600,000
Accident Year 2013 | Property Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 110,700,000 112,800,000 112,600,000 112,300,000 114,300,000 113,300,000 117,500,000 117,900,000 130,800,000  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 300,000                  
Number of Reported Claims | Integer 5,764                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 110,000,000.0 111,200,000 110,900,000 109,100,000 106,400,000 101,700,000 89,700,000 76,400,000 39,200,000  
Accident Year 2013 | Casualty Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 100,900,000 104,300,000 105,700,000 105,400,000 104,300,000 121,300,000 115,800,000 117,400,000 134,000,000.0  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 7,200,000                  
Number of Reported Claims | Integer 3,367                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 88,000,000.0 87,800,000 85,800,000 81,500,000 69,000,000.0 53,400,000 39,800,000 26,000,000.0 2,300,000  
Accident Year 2013 | Marine, Aviation and Energy Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 343,300,000 340,200,000 345,200,000 346,600,000 333,000,000.0 325,900,000 342,500,000 333,800,000 321,000,000.0  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 2,800,000                  
Number of Reported Claims | Integer 4,178                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 311,000,000.0 310,800,000 300,600,000 284,500,000 264,800,000 235,400,000 205,000,000.0 131,700,000 41,500,000  
Accident Year 2013 | Financial and Professional Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 102,800,000 97,600,000 90,900,000 91,900,000 100,800,000 101,700,000 105,000,000.0 100,600,000 106,100,000  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 2,200,000                  
Number of Reported Claims | Integer 586                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 81,800,000 78,300,000 75,000,000.0 72,800,000 64,100,000 65,600,000 31,300,000 21,200,000 8,100,000  
Accident Year 2013 | Property Catastrophe and Other Property Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 164,700,000 168,200,000 171,000,000.0 173,800,000 177,100,000 178,500,000 189,700,000 199,900,000 217,400,000  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 600,000                  
Number of Reported Claims | Integer 832                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 161,200,000 161,200,000 166,800,000 165,200,000 163,700,000 159,000,000.0 146,900,000 98,800,000 34,500,000  
Accident Year 2013 | Casualty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 165,800,000 201,600,000 204,400,000 201,700,000 206,600,000 223,600,000 226,300,000 230,800,000 215,600,000  
Incurred but Not Reported (IBNR) Claims Liability, Net $ (6,800,000)                  
Number of Reported Claims | Integer 1,691                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 146,300,000 139,900,000 127,800,000 115,000,000.0 93,100,000 65,100,000 42,800,000 15,900,000 3,400,000  
Accident Year 2013 | Specialty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 104,800,000 111,000,000.0 114,400,000 114,800,000 118,600,000 119,400,000 131,300,000 139,600,000 144,400,000  
Incurred but Not Reported (IBNR) Claims Liability, Net $ (500,000)                  
Number of Reported Claims | Integer 577                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 100,700,000 101,700,000 101,900,000 101,000,000.0 101,000,000.0 94,000,000.0 86,900,000 70,900,000 $ 25,000,000.0  
Accident Year 2014 | Property Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 133,400,000 132,400,000 133,000,000.0 134,600,000 135,300,000 134,700,000 157,900,000 166,100,000    
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 9,973                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 132,300,000 131,000,000.0 129,800,000 128,300,000 124,200,000 114,500,000 86,900,000 40,500,000    
Accident Year 2014 | Casualty Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 130,400,000 138,800,000 141,800,000 137,700,000 130,500,000 140,300,000 128,400,000 146,200,000    
Incurred but Not Reported (IBNR) Claims Liability, Net $ 5,700,000                  
Number of Reported Claims | Integer 3,865                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 110,100,000 111,400,000 98,400,000 74,000,000.0 60,400,000 33,100,000 13,500,000 2,700,000    
Accident Year 2014 | Marine, Aviation and Energy Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 312,500,000 302,200,000 313,000,000.0 306,000,000.0 310,500,000 298,900,000 314,000,000.0 309,800,000    
Incurred but Not Reported (IBNR) Claims Liability, Net $ (5,900,000)                  
Number of Reported Claims | Integer 4,043                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 275,600,000 263,100,000 250,900,000 232,700,000 210,000,000.0 189,300,000 116,900,000 53,600,000    
Accident Year 2014 | Financial and Professional Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 123,700,000 121,700,000 120,900,000 131,900,000 120,600,000 130,200,000 131,600,000 135,700,000    
Incurred but Not Reported (IBNR) Claims Liability, Net $ (2,700,000)                  
Number of Reported Claims | Integer 800                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 104,200,000 92,600,000 85,900,000 80,000,000.0 72,500,000 53,800,000 30,900,000 3,000,000.0    
Accident Year 2014 | Property Catastrophe and Other Property Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 143,900,000 144,900,000 144,900,000 149,800,000 149,500,000 160,900,000 176,800,000 189,900,000    
Incurred but Not Reported (IBNR) Claims Liability, Net $ 800,000                  
Number of Reported Claims | Integer 900                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 141,500,000 141,300,000 139,600,000 140,900,000 137,000,000.0 127,100,000 100,700,000 37,400,000    
Accident Year 2014 | Casualty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 160,500,000 202,900,000 207,100,000 204,500,000 211,100,000 217,600,000 209,000,000.0 206,200,000    
Incurred but Not Reported (IBNR) Claims Liability, Net $ (14,800,000)                  
Number of Reported Claims | Integer 1,805                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 135,700,000 125,700,000 107,700,000 86,700,000 60,300,000 37,800,000 13,800,000 2,500,000    
Accident Year 2014 | Specialty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 114,900,000 120,100,000 124,300,000 125,800,000 122,900,000 132,100,000 140,100,000 152,400,000    
Incurred but Not Reported (IBNR) Claims Liability, Net $ 600,000                  
Number of Reported Claims | Integer 619                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 106,200,000 104,200,000 102,600,000 99,700,000 89,300,000 81,300,000 56,500,000 $ 16,600,000    
Accident Year 2015 | Property Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 202,200,000 201,500,000 204,600,000 204,200,000 201,800,000 207,400,000 241,800,000      
Incurred but Not Reported (IBNR) Claims Liability, Net $ 2,000,000.0                  
Number of Reported Claims | Integer 11,598                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 197,800,000 196,400,000 198,000,000.0 181,300,000 172,100,000 144,100,000 57,500,000      
Accident Year 2015 | Casualty Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 227,700,000 236,700,000 238,400,000 205,500,000 187,600,000 225,100,000 205,100,000      
Incurred but Not Reported (IBNR) Claims Liability, Net $ 13,500,000                  
Number of Reported Claims | Integer 4,736                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 181,400,000 169,200,000 139,700,000 93,500,000 57,000,000.0 17,200,000 3,200,000      
Accident Year 2015 | Marine, Aviation and Energy Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 321,400,000 313,000,000.0 310,400,000 286,800,000 282,600,000 300,200,000 297,100,000      
Incurred but Not Reported (IBNR) Claims Liability, Net $ (4,600,000)                  
Number of Reported Claims | Integer 4,074                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 280,600,000 257,900,000 222,500,000 194,500,000 174,600,000 123,400,000 44,900,000      
Accident Year 2015 | Financial and Professional Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 185,900,000 186,700,000 191,900,000 190,600,000 186,400,000 176,300,000 175,000,000.0      
Incurred but Not Reported (IBNR) Claims Liability, Net $ (400,000)                  
Number of Reported Claims | Integer 1,080                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 146,200,000 139,600,000 110,400,000 89,700,000 70,300,000 43,600,000 13,800,000      
Accident Year 2015 | Property Catastrophe and Other Property Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 179,800,000 171,900,000 171,800,000 156,600,000 177,200,000 187,300,000 214,600,000      
Incurred but Not Reported (IBNR) Claims Liability, Net $ 11,700,000                  
Number of Reported Claims | Integer 1,051                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 162,200,000 158,600,000 156,200,000 139,200,000 127,100,000 95,000,000.0 35,800,000      
Accident Year 2015 | Casualty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 160,600,000 208,000,000.0 212,000,000.0 214,200,000 211,700,000 202,200,000 195,400,000      
Incurred but Not Reported (IBNR) Claims Liability, Net $ (13,600,000)                  
Number of Reported Claims | Integer 1,964                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 123,300,000 109,000,000.0 89,700,000 65,700,000 38,500,000 18,000,000.0 3,500,000      
Accident Year 2015 | Specialty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 149,000,000.0 152,800,000 157,300,000 159,000,000.0 164,500,000 170,000,000.0 165,900,000      
Incurred but Not Reported (IBNR) Claims Liability, Net $ 4,700,000                  
Number of Reported Claims | Integer 777                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 134,000,000.0 134,000,000.0 130,900,000 122,000,000.0 104,100,000 56,400,000 $ 17,700,000      
Accident Year 2016 | Property Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 249,200,000 248,400,000 246,900,000 245,500,000 250,700,000 239,900,000        
Incurred but Not Reported (IBNR) Claims Liability, Net $ 1,600,000                  
Number of Reported Claims | Integer 10,785                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 239,000,000.0 233,800,000 224,800,000 202,500,000 169,700,000 67,300,000        
Accident Year 2016 | Casualty Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 233,800,000 203,100,000 192,000,000.0 185,300,000 190,000,000.0 218,900,000        
Incurred but Not Reported (IBNR) Claims Liability, Net $ 54,100,000                  
Number of Reported Claims | Integer 4,753                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 134,800,000 111,000,000.0 83,500,000 40,700,000 23,100,000 4,200,000        
Accident Year 2016 | Marine, Aviation and Energy Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 221,700,000 219,400,000 229,600,000 229,400,000 230,800,000 260,800,000        
Incurred but Not Reported (IBNR) Claims Liability, Net $ 200,000                  
Number of Reported Claims | Integer 4,429                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 199,000,000.0 191,200,000 164,300,000 142,800,000 82,700,000 30,900,000        
Accident Year 2016 | Financial and Professional Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 185,700,000 186,300,000 203,100,000 217,400,000 212,600,000 191,900,000        
Incurred but Not Reported (IBNR) Claims Liability, Net $ 13,500,000                  
Number of Reported Claims | Integer 1,255                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 136,000,000.0 126,600,000 130,600,000 102,500,000 71,400,000 15,200,000        
Accident Year 2016 | Property Catastrophe and Other Property Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 230,400,000 244,900,000 249,200,000 271,000,000.0 272,700,000 272,600,000        
Incurred but Not Reported (IBNR) Claims Liability, Net $ (15,200,000)                  
Number of Reported Claims | Integer 1,308                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 236,700,000 230,400,000 217,100,000 206,000,000.0 164,700,000 57,200,000        
Accident Year 2016 | Casualty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 213,500,000 264,700,000 257,200,000 247,000,000.0 247,300,000 234,700,000        
Incurred but Not Reported (IBNR) Claims Liability, Net $ (2,100,000)                  
Number of Reported Claims | Integer 2,056                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 148,000,000.0 127,000,000.0 96,700,000 64,500,000 33,700,000 9,300,000        
Accident Year 2016 | Specialty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 214,700,000 225,000,000.0 230,000,000.0 237,700,000 239,900,000 238,800,000        
Incurred but Not Reported (IBNR) Claims Liability, Net $ (5,400,000)                  
Number of Reported Claims | Integer 937                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 203,800,000 193,800,000 183,600,000 165,500,000 150,900,000 $ 58,700,000        
Accident Year 2017 | Property Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 254,100,000 254,900,000 253,500,000 260,300,000 297,500,000          
Incurred but Not Reported (IBNR) Claims Liability, Net $ 10,300,000                  
Number of Reported Claims | Integer 9,693                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 248,400,000 244,100,000 223,200,000 189,900,000 97,000,000.0          
Accident Year 2017 | Casualty Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 201,600,000 199,300,000 180,400,000 176,300,000 182,900,000          
Incurred but Not Reported (IBNR) Claims Liability, Net $ 18,900,000                  
Number of Reported Claims | Integer 5,416                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 116,800,000 98,400,000 53,400,000 23,200,000 3,600,000          
Accident Year 2017 | Marine, Aviation and Energy Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 209,800,000 214,900,000 207,400,000 201,100,000 210,700,000          
Incurred but Not Reported (IBNR) Claims Liability, Net $ 37,900,000                  
Number of Reported Claims | Integer 6,041                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 156,900,000 168,800,000 140,600,000 97,800,000 40,300,000          
Accident Year 2017 | Financial and Professional Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 188,600,000 189,400,000 188,900,000 184,100,000 207,700,000          
Incurred but Not Reported (IBNR) Claims Liability, Net $ 12,800,000                  
Number of Reported Claims | Integer 1,739                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 137,200,000 117,500,000 83,600,000 51,400,000 27,200,000          
Accident Year 2017 | Property Catastrophe and Other Property Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 481,200,000 505,000,000.0 516,200,000 534,500,000 557,700,000          
Incurred but Not Reported (IBNR) Claims Liability, Net $ (3,800,000)                  
Number of Reported Claims | Integer 1,956                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 437,600,000 440,800,000 416,600,000 357,400,000 123,200,000          
Accident Year 2017 | Casualty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 194,200,000 254,100,000 254,900,000 244,000,000.0 246,600,000          
Incurred but Not Reported (IBNR) Claims Liability, Net $ 3,400,000                  
Number of Reported Claims | Integer 2,005                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 117,300,000 97,900,000 59,200,000 30,700,000 8,900,000          
Accident Year 2017 | Specialty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 343,200,000 364,400,000 376,000,000.0 392,400,000 379,700,000          
Incurred but Not Reported (IBNR) Claims Liability, Net $ (6,100,000)                  
Number of Reported Claims | Integer 1,328                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 317,100,000 306,300,000 271,000,000.0 238,800,000 $ 94,600,000          
Accident Year 2018 | Property Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 190,400,000 191,900,000 208,200,000 205,500,000            
Incurred but Not Reported (IBNR) Claims Liability, Net $ 400,000                  
Number of Reported Claims | Integer 8,171                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 181,800,000 185,000,000.0 162,900,000 62,600,000            
Accident Year 2018 | Casualty Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 135,500,000 137,900,000 126,800,000 124,000,000.0            
Incurred but Not Reported (IBNR) Claims Liability, Net $ 22,700,000                  
Number of Reported Claims | Integer 5,419                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 69,700,000 44,100,000 28,200,000 3,200,000            
Accident Year 2018 | Marine, Aviation and Energy Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 219,900,000 209,400,000 208,500,000 171,400,000            
Incurred but Not Reported (IBNR) Claims Liability, Net $ (2,400,000)                  
Number of Reported Claims | Integer 5,178                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 155,700,000 133,400,000 105,100,000 26,900,000            
Accident Year 2018 | Financial and Professional Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 152,700,000 156,900,000 174,600,000 158,500,000            
Incurred but Not Reported (IBNR) Claims Liability, Net $ 6,000,000.0                  
Number of Reported Claims | Integer 4,617                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 117,100,000 101,600,000 75,700,000 21,000,000.0            
Accident Year 2018 | Property Catastrophe and Other Property Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 403,500,000 389,500,000 394,200,000 343,100,000            
Incurred but Not Reported (IBNR) Claims Liability, Net $ 22,200,000                  
Number of Reported Claims | Integer 1,808                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 345,400,000 325,700,000 311,600,000 122,800,000            
Accident Year 2018 | Casualty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 190,600,000 267,000,000.0 259,800,000 230,100,000            
Incurred but Not Reported (IBNR) Claims Liability, Net $ 24,700,000                  
Number of Reported Claims | Integer 1,766                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 104,900,000 73,700,000 33,700,000 7,200,000            
Accident Year 2018 | Specialty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 391,100,000 394,300,000 396,400,000 397,400,000            
Incurred but Not Reported (IBNR) Claims Liability, Net $ 13,000,000.0                  
Number of Reported Claims | Integer 1,399                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 331,100,000 314,600,000 280,800,000 $ 27,200,000            
Accident Year 2019 | Property Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 122,600,000 132,100,000 128,500,000              
Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Number of Reported Claims | Integer 6,812                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 103,400,000 92,600,000 49,600,000              
Accident Year 2019 | Casualty Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 127,900,000 148,800,000 126,500,000              
Incurred but Not Reported (IBNR) Claims Liability, Net $ 4,700,000                  
Number of Reported Claims | Integer 5,104                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 70,400,000 18,600,000 6,400,000              
Accident Year 2019 | Marine, Aviation and Energy Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 146,000,000.0 154,100,000 146,500,000              
Incurred but Not Reported (IBNR) Claims Liability, Net $ 3,000,000.0                  
Number of Reported Claims | Integer 3,575                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 118,100,000 72,800,000 33,600,000              
Accident Year 2019 | Financial and Professional Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 236,000,000.0 263,800,000 250,500,000              
Incurred but Not Reported (IBNR) Claims Liability, Net $ 52,800,000                  
Number of Reported Claims | Integer 23,894                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 138,400,000 87,300,000 27,400,000              
Accident Year 2019 | Property Catastrophe and Other Property Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 270,600,000 272,300,000 262,800,000              
Incurred but Not Reported (IBNR) Claims Liability, Net $ 3,600,000                  
Number of Reported Claims | Integer 1,395                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 205,600,000 153,000,000.0 28,200,000              
Accident Year 2019 | Casualty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 179,200,000 256,600,000 236,200,000              
Incurred but Not Reported (IBNR) Claims Liability, Net $ 52,300,000                  
Number of Reported Claims | Integer 1,300                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 61,400,000 36,500,000 9,200,000              
Accident Year 2019 | Specialty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 495,400,000 498,900,000 476,100,000              
Incurred but Not Reported (IBNR) Claims Liability, Net $ 49,200,000                  
Number of Reported Claims | Integer 1,519                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 402,100,000 382,700,000 $ 274,300,000              
Accident Year 2020 | Property Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 211,200,000 206,700,000                
Incurred but Not Reported (IBNR) Claims Liability, Net $ 36,200,000                  
Number of Reported Claims 7,295                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 125,500,000 61,900,000                
Accident Year 2020 | Casualty Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 133,900,000 134,900,000                
Incurred but Not Reported (IBNR) Claims Liability, Net $ 94,300,000                  
Number of Reported Claims 3,570                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 3,200,000 0                
Accident Year 2020 | Marine, Aviation and Energy Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 111,800,000 111,000,000.0                
Incurred but Not Reported (IBNR) Claims Liability, Net $ 12,000,000.0                  
Number of Reported Claims | Integer 4,128                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 66,700,000 28,500,000                
Accident Year 2020 | Financial and Professional Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 352,000,000.0 351,000,000.0                
Incurred but Not Reported (IBNR) Claims Liability, Net $ 181,700,000                  
Number of Reported Claims | Integer 105,317                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 121,900,000 48,000,000.0                
Accident Year 2020 | Property Catastrophe and Other Property Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 366,900,000 322,900,000                
Incurred but Not Reported (IBNR) Claims Liability, Net $ 20,100,000                  
Number of Reported Claims | Integer 1,333                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 169,200,000 42,400,000                
Accident Year 2020 | Casualty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 237,600,000 256,400,000                
Incurred but Not Reported (IBNR) Claims Liability, Net $ 177,800,000                  
Number of Reported Claims | Integer 753                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 28,000,000.0 9,200,000                
Accident Year 2020 | Specialty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 382,400,000 415,600,000                
Incurred but Not Reported (IBNR) Claims Liability, Net $ 66,700,000                  
Number of Reported Claims | Integer 1,400                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 270,400,000 $ 213,100,000                
Accident Year 2021 | Property Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 210,900,000                  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 88,800,000                  
Number of Reported Claims 4,978                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 59,200,000                  
Accident Year 2021 | Casualty Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 176,500,000                  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 150,500,000                  
Number of Reported Claims 2,707                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 9,500,000                  
Accident Year 2021 | Marine, Aviation and Energy Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 94,000,000.0                  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 44,700,000                  
Number of Reported Claims | Integer 2,108                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 23,500,000                  
Accident Year 2021 | Financial and Professional Insurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 289,600,000                  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 221,500,000                  
Number of Reported Claims | Integer 31,311                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 43,700,000                  
Accident Year 2021 | Property Catastrophe and Other Property Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 461,500,000                  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 198,600,000                  
Number of Reported Claims | Integer 1,005                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 75,600,000                  
Accident Year 2021 | Casualty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 209,000,000.0                  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 177,000,000.0                  
Number of Reported Claims | Integer 355                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 7,900,000                  
Accident Year 2021 | Specialty Reinsurance                    
Claims Development [Line Items]                    
Incurred Claims and Allocated Claim Adjustment Expense, Net 155,800,000                  
Incurred but Not Reported (IBNR) Claims Liability, Net $ 106,100,000                  
Number of Reported Claims | Integer 1,020                  
Cumulative paid claims and allocated claim adjustment expense, Net $ 28,400,000