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Segment Information (Reconciliation of Segment Results to Income Before Federal Income Tax (Details)) - USD ($) $ in Thousands | 3 Months Ended
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Mar. 31, 2021 | Mar. 31, 2020
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |
Investment income | $ 74,835 | $ 11,301
Interest expense | (7,359) | (7,601)
Corporate expenses | (9,554) | (9,060)
Income before federal income tax | 135,632 | 15,997
Preferred stock dividends | (2,453) | 0
Income available to common stockholders, before tax | 133,179 | 15,997
Standard Commercial Lines |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |
Underwriting gain, before federal income tax | 69,499 | 17,126
Standard Personal Lines |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |
Underwriting gain, before federal income tax | 7,695 | 387
E&S Lines |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |
Underwriting gain, before federal income tax | 516 | 3,844
Operating Segments |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |
Total income, before federal income tax all segments | $ 152,545 | $ 32,658

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