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Segment Information - Summary of net premiums written and earned by major line of business and net premiums written by location (Details) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2019
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Segment Reporting Information [Line Items]                      
Net premiums earned $ 1,515,882 $ 1,438,023 $ 1,463,727 $ 1,368,866 $ 1,369,435 $ 1,290,878 $ 1,336,763 $ 1,234,899 $ 5,786,498 $ 5,231,975 $ 4,844,532
Net premiums written $ 1,455,453 $ 1,613,457 $ 1,444,898 $ 1,525,259 $ 1,301,754 $ 1,333,553 $ 1,298,896 $ 1,412,544 6,039,067 5,346,747 4,961,373
Other                      
Segment Reporting Information [Line Items]                      
Net premiums earned                 556,689 578,862 531,727
Net premiums written                 532,862 604,175 553,117
Other | United States                      
Segment Reporting Information [Line Items]                      
Net premiums written                 127,176 49,800 11,750
Other | Europe                      
Segment Reporting Information [Line Items]                      
Net premiums written                 52,065 91,635 91,463
Other | Bermuda                      
Segment Reporting Information [Line Items]                      
Net premiums written                 353,621 462,740 449,904
Other | Other specialty                      
Segment Reporting Information [Line Items]                      
Net premiums earned [1],[2]                 185,547 204,485 135,855
Other | Casualty                      
Segment Reporting Information [Line Items]                      
Net premiums earned [2],[3]                 246,894 277,589 333,275
Other | Property excluding property catastrophe                      
Segment Reporting Information [Line Items]                      
Net premiums earned [2]                 3,503 2,802 1,392
Other | Property catastrophe                      
Segment Reporting Information [Line Items]                      
Net premiums earned [2]                 13,399 10,998 12,690
Other | Other                      
Segment Reporting Information [Line Items]                      
Net premiums earned [2],[4]                 107,346 82,988 48,515
Underwriting segments                      
Segment Reporting Information [Line Items]                      
Net premiums earned                 5,229,809 4,653,113 4,312,805
Net premiums written                 5,506,205 4,742,572 4,408,256
Underwriting segments | Insurance                      
Segment Reporting Information [Line Items]                      
Net premiums earned                 2,397,080 2,205,661 2,113,018
Net premiums written                 2,641,726 2,212,125 2,122,440
Underwriting segments | Insurance | United States                      
Segment Reporting Information [Line Items]                      
Net premiums written [5]                 1,983,476 1,736,651 1,715,467
Underwriting segments | Insurance | Europe                      
Segment Reporting Information [Line Items]                      
Net premiums written [5]                 559,214 401,974 344,836
Underwriting segments | Insurance | Other geographic location                      
Segment Reporting Information [Line Items]                      
Net premiums written [5]                 99,036 73,500 62,137
Underwriting segments | Insurance | Professional lines                      
Segment Reporting Information [Line Items]                      
Net premiums earned [6]                 499,224 458,425 444,137
Underwriting segments | Insurance | Programs                      
Segment Reporting Information [Line Items]                      
Net premiums earned [5]                 414,103 389,186 364,639
Underwriting segments | Insurance | Property, energy, marine and aviation                      
Segment Reporting Information [Line Items]                      
Net premiums earned [5]                 298,966 205,069 173,779
Underwriting segments | Insurance | Construction and national accounts                      
Segment Reporting Information [Line Items]                      
Net premiums earned [5],[7]                 325,687 322,440 324,517
Underwriting segments | Insurance | Travel, accident and health                      
Segment Reporting Information [Line Items]                      
Net premiums earned [5]                 305,085 297,147 257,358
Underwriting segments | Insurance | Excess and surplus casualty                      
Segment Reporting Information [Line Items]                      
Net premiums earned [5]                 200,615 172,424 195,154
Underwriting segments | Insurance | Lenders products                      
Segment Reporting Information [Line Items]                      
Net premiums earned [5]                 66,079 94,248 97,043
Underwriting segments | Insurance | Other                      
Segment Reporting Information [Line Items]                      
Net premiums earned [5],[8]                 287,321 266,722 256,391
Underwriting segments | Reinsurance                      
Segment Reporting Information [Line Items]                      
Net premiums earned                 1,466,389 1,261,216 1,142,621
Net premiums written                 1,602,723 1,372,572 1,174,474
Underwriting segments | Reinsurance | United States                      
Segment Reporting Information [Line Items]                      
Net premiums written [9]                 529,943 413,550 399,379
Underwriting segments | Reinsurance | Bermuda                      
Segment Reporting Information [Line Items]                      
Net premiums written [9]                 578,618 487,523 350,681
Underwriting segments | Reinsurance | Europe and other                      
Segment Reporting Information [Line Items]                      
Net premiums written                 494,162 471,499 424,414
Underwriting segments | Reinsurance | Other specialty                      
Segment Reporting Information [Line Items]                      
Net premiums earned [9],[10]                 478,517 474,568 408,566
Underwriting segments | Reinsurance | Casualty                      
Segment Reporting Information [Line Items]                      
Net premiums earned [9],[11]                 429,288 347,034 341,122
Underwriting segments | Reinsurance | Property excluding property catastrophe                      
Segment Reporting Information [Line Items]                      
Net premiums earned [9],[12]                 362,841 287,788 255,453
Underwriting segments | Reinsurance | Property catastrophe                      
Segment Reporting Information [Line Items]                      
Net premiums earned [9]                 90,934 75,249 73,300
Underwriting segments | Reinsurance | Marine and aviation                      
Segment Reporting Information [Line Items]                      
Net premiums earned [9]                 48,274 39,238 36,214
Underwriting segments | Reinsurance | Other                      
Segment Reporting Information [Line Items]                      
Net premiums earned [9],[13]                 56,535 37,339 27,966
Underwriting segments | Mortgage                      
Segment Reporting Information [Line Items]                      
Net premiums earned                 1,366,340 1,186,236 1,057,166
Net premiums written                 1,261,756 1,157,875 1,111,342
Underwriting segments | Mortgage | United States                      
Segment Reporting Information [Line Items]                      
Net premiums earned                 1,134,849 1,009,765 901,858
Net premiums written                 1,032,868 948,323 903,329
Underwriting segments | Mortgage | Other geographic location                      
Segment Reporting Information [Line Items]                      
Net premiums earned                 231,491 176,471 155,308
Net premiums written                 $ 228,888 $ 209,552 $ 208,013
[1]
Includes proportional motor and other.
[2]
Other segment results include premiums assumed through intersegment transactions and exclude premiums ceded through intersegment transactions.
[3]
Includes professional liability, excess motor, programs and other.
[4]
Includes mortgage and other.
[5]
Insurance segment results include premiums assumed through intersegment transactions and exclude premiums ceded through intersegment transactions.
[6]
Includes professional liability, executive assurance and healthcare business.
[7]
Includes casualty and contract binding business.
[8]
Includes alternative markets, excess workers' compensation and surety business.
[9]
Reinsurance segment results include premiums assumed through intersegment transactions and exclude premiums ceded through intersegment transactions.
[10] Includes proportional motor, surety, accident and health, workers’ compensation catastrophe, agriculture, trade credit and other.
[11]
Includes executive assurance, professional liability, workers’ compensation, excess motor, healthcare and other.
[12]
Includes life, casualty clash and other.
[13]
Includes life, casualty clash and other.