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Segment Information (Reconciliation of Segment Results to Income from Continuing Operations, before Federal Income Tax (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, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total investment revenues |  |  |  |  |  |  |  |  | $ 236,965 | $ 140,413 | $ 168,241
Interest expense |  |  |  |  |  |  |  |  | (33,668) | (24,419) | (24,354)
Corporate expenses |  |  |  |  |  |  |  |  | (30,900) | (25,446) | (36,255)
Income before federal income tax | $ 101,293 | $ 71,178 | $ 90,225 | $ 73,694 | $ 52,135 | $ 67,130 | $ 72,525 | $ 19,931 | 336,390 | 211,721 | 261,968
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 |  |  |  |  |  |  |  |  | 145,990 | 109,104 | 149,514
Standard Personal Lines [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |  |  |  |  |  |  |  |  |
Underwriting gain (loss), before federal income tax |  |  |  |  |  |  |  |  | 8,260 | 12,764 | 11,104
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 |  |  |  |  |  |  |  |  | 9,743 | (695) | (6,282)
Operating Segments [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total all segments |  |  |  |  |  |  |  |  | $ 400,958 | $ 261,586 | $ 322,577

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