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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, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2015 | Sep. 30, 2015 | Jun. 30, 2015 | Mar. 31, 2015 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |  |  |  |  |  |  |  |  |
Underwriting gain (loss), before federal income tax | $ 35,168 | $ 32,033 | $ 43,777 | $ 40,955 | $ 49,053 | $ 44,831 | $ 29,124 | $ 26,021 |  |  |
Total investment revenues |  |  |  |  |  |  |  |  | $ 125,817 | $ 134,487 | $ 165,307
Interest expense |  |  |  |  |  |  |  |  | (22,771) | (22,428) | (23,063)
General corporate and other expenses |  |  |  |  |  |  |  |  | (35,024) | (28,396) | (23,256)
Income from continuing operations, before federal income tax |  |  |  |  |  |  |  |  | 219,955 | 232,692 | 197,131
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 |  |  |  |  |  |  |  |  | 146,435 | 164,496 | 61,221
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 |  |  |  |  |  |  |  |  | 12,419 | 1,336 | 16,536
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 |  |  |  |  |  |  |  |  | (6,921) | (16,803) | 386
Operating Segments [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total all segments |  |  |  |  |  |  |  |  | $ 277,750 | $ 283,516 | $ 243,450

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