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Segment Information (Reconciliation of Segment Results to Income Before Federal Income Tax (Details)) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2015 | Sep. 30, 2014 | Sep. 30, 2015 | Sep. 30, 2014
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |  |
Investment income, before federal income tax | $ 32,369 | $ 49,523 | $ 106,979 | $ 133,588
Interest expense | (5,489) | (5,558) | (16,458) | (16,544)
General corporate and other expenses | (5,872) | (3,076) | (21,603) | (16,619)
Income before federal income tax | 65,839 | 75,326 | 168,894 | 139,931
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 | 44,027 | 27,771 | 109,304 | 39,844
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 | 2,826 | 8,037 | (4,295) | 95
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 | (2,022) | (1,371) | (5,033) | (433)
Operating Segments [Member] |  |  |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |  |
Total all segments | $ 77,200 | $ 83,960 | $ 206,955 | $ 173,094

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