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Business Segments (Financial Information of Company Operating Segments) (Details) $ in Thousands | 3 Months Ended
--- | ---
Mar. 31, 2024 USD ($) segment | Mar. 31, 2023 USD ($)
Segment Reporting [Abstract] |  |
Number of reportable segments | segment | 2 |
Segment Reporting Information [Line Items] |  |
Earned Premiums (1) | $ 2,764,347 | $ 2,491,432
Investment Income | 319,839 | 223,398
Other | 172,587 | 180,174
Total revenues | 3,256,773 | 2,895,004
Pre-Tax Income (Loss) | 574,071 | 376,071
Net Income (Loss) to Common Stockholders | 442,471 | 294,126
Net investment gains | 25,780 | 23,010
Net investment gains, net of tax | 19,147 | 18,160
Corporate, other and eliminations |  |
Segment Reporting Information [Line Items] |  |
Earned Premiums (1) | 0 | 0
Investment Income | 21,850 | 5,257
Other | 137,384 | 147,587
Total revenues | 159,234 | 152,844
Pre-Tax Income (Loss) | (57,482) | (100,850)
Net Income (Loss) to Common Stockholders | (43,893) | (80,499)
Insurance |  |
Segment Reporting Information [Line Items] |  |
Earned Premiums (1) |  | 2,142,924
Insurance | Operating Segments |  |
Segment Reporting Information [Line Items] |  |
Earned Premiums (1) | 2,398,768 | 2,142,924
Investment Income | 244,778 | 163,558
Other | 9,423 | 9,577
Total revenues | 2,652,969 | 2,316,059
Pre-Tax Income (Loss) | 478,149 | 352,626
Net Income (Loss) to Common Stockholders | 365,091 | 274,108
Insurance | Operating Segments | Non-US |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 393,000 | 274,000
Reinsurance & Monoline Excess |  |
Segment Reporting Information [Line Items] |  |
Earned Premiums (1) |  | 348,508
Reinsurance & Monoline Excess | Operating Segments |  |
Segment Reporting Information [Line Items] |  |
Earned Premiums (1) | 365,579 | 348,508
Investment Income | 53,211 | 54,583
Other | 0 | 0
Total revenues | 418,790 | 403,091
Pre-Tax Income (Loss) | 127,624 | 101,285
Net Income (Loss) to Common Stockholders | 102,126 | 82,357
Reinsurance & Monoline Excess | Operating Segments | Non-US |  |
Segment Reporting Information [Line Items] |  |
Total revenues | 111,000 | 106,000
Net investment gains |  |
Segment Reporting Information [Line Items] |  |
Earned Premiums (1) | 0 | 0
Investment Income | $ 0 | $ 0

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