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Segment Information (Reconciliation of Segment Results to Income Before Federal Income Tax (Details)) - USD ($) $ in Thousands | 3 Months Ended
--- | ---
Mar. 31, 2016 | Mar. 31, 2015
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |
Investment income, before federal income tax | $ 28,065 | $ 45,800
Interest expense | (5,606) | (5,604)
General corporate and other expenses | (11,539) | (9,927)
Income before federal income tax | 51,875 | 56,290
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 | 30,932 | 30,036
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 | 8,605 | (2,466)
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 | 1,418 | (1,549)
Operating Segments [Member] |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |
Total all segments | $ 69,020 | $ 71,821

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