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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, 2017 | Sep. 30, 2016 | Sep. 30, 2017 | Sep. 30, 2016
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |  |
Investment income, before federal income tax | $ 47,244 | $ 37,063 | $ 126,782 | $ 98,075
Interest expense | (6,085) | (5,714) | (18,272) | (16,940)
General corporate and other expenses | (6,289) | (7,939) | (26,669) | (28,271)
Income before federal income tax | 67,315 | 55,443 | 193,818 | 169,629
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 | 35,329 | 30,124 | 112,634 | 101,229
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 | 8,179 | 4,271 | 7,517 | 19,001
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 | (11,063) | (2,362) | (8,174) | (3,465)
Operating Segments [Member] |  |  |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |  |
Total all segments | $ 79,689 | $ 69,096 | $ 238,759 | $ 214,840

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