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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, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total investment revenues |  |  |  |  |  |  |  |  | $ 168,241 | $ 125,817 | $ 134,487
Interest expense |  |  |  |  |  |  |  |  | (24,354) | (22,771) | (22,428)
Corporate expenses |  |  |  |  |  |  |  |  | (36,255) | (35,024) | (28,396)
Income before federal income tax | $ 68,150 | $ 67,315 | $ 58,929 | $ 67,574 | $ 50,326 | $ 55,443 | $ 62,311 | $ 51,875 | 261,968 | 219,955 | 232,692
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 |  |  |  |  |  |  |  |  | 149,514 | 146,435 | 164,496
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 |  |  |  |  |  |  |  |  | 11,104 | 12,419 | 1,336
Excess And Surplus Operations [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |  |  |  |  |  |  |  |  |
Underwriting gain (loss), before federal income tax |  |  |  |  |  |  |  |  | (6,282) | (6,921) | (16,803)
Operating Segments [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total all segments |  |  |  |  |  |  |  |  | $ 322,577 | $ 277,750 | $ 283,516

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