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Segment Information (Reconciliation of Segment Results to Income from Continuing Operations, before Federal Income Tax) (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |
Total investment revenues | $ 344,188 | $ 222,890 | $ 236,965
Interest expense | (29,165) | (30,839) | (33,668)
Corporate expenses | (28,305) | (25,412) | (30,900)
Income before federal income tax | 505,310 | 302,988 | 336,390
Preferred Stock Dividends, Income Statement Impact | (9,353) | 0 | 0
Income available to common stockholders, before tax | 495,957 | 302,988 | 336,390
Standard Commercial Lines [Member] |  |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |
Underwriting gain (loss), before federal income tax | 198,596 | 151,731 | 145,990
Standard Personal Lines |  |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |
Underwriting gain (loss), before federal income tax | 3,966 | (15,508) | 8,260
E&S Lines [Member] |  |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |
Underwriting gain (loss), before federal income tax | 16,030 | 126 | 9,743
Operating Segments [Member] |  |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |
Total all segments | $ 562,780 | $ 359,239 | $ 400,958

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