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Segment Information (Reconciliation of Segment Results to Income Before Federal Income Tax (Details)) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2015 | Jun. 30, 2014 | Jun. 30, 2015 | Jun. 30, 2014
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |  |
Investment income, before federal income tax | $ 28,810 | $ 41,313 | $ 74,610 | $ 84,065
Interest expense | (5,490) | (5,425) | (10,969) | (10,986)
General corporate and other expenses | (5,679) | (6,451) | (15,731) | (13,543)
Income before federal income tax | 46,765 | 39,521 | 103,055 | 64,605
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 | 35,241 | 15,703 | 65,277 | 12,073
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 | (4,655) | (5,582) | (7,121) | (7,942)
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 | (1,462) | (37) | (3,011) | 938
Operating Segments [Member] |  |  |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |  |
Total all segments | $ 57,934 | $ 51,397 | $ 129,755 | $ 89,134

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