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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, 2016 | Sep. 30, 2015 | Sep. 30, 2016 | Sep. 30, 2015
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |  |
Investment income, before federal income tax | $ 37,063 | $ 32,369 | $ 98,075 | $ 106,979
Interest expense | (5,714) | (5,610) | (16,940) | (16,826)
General corporate and other expenses | (7,939) | (5,751) | (28,271) | (21,235)
Income before federal income tax | 55,443 | 65,839 | 169,629 | 168,894
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 | 30,124 | 44,027 | 101,229 | 109,304
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 | 4,271 | 2,826 | 19,001 | (4,295)
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 | (2,362) | (2,022) | (3,465) | (5,033)
Operating Segments [Member] |  |  |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |  |
Total all segments | $ 69,096 | $ 77,200 | $ 214,840 | $ 206,955

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