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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, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |  |
Investment income, before federal income tax | $ 47,656 | $ 47,244 | $ 124,239 | $ 126,782
Interest expense | (6,073) | (6,085) | (18,350) | (18,272)
Corporate expenses | (7,450) | (6,289) | (22,065) | (26,669)
Income before federal income tax | 67,130 | 67,315 | 159,586 | 193,818
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 | 26,333 | 35,329 | 74,153 | 112,634
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 | 3,158 | 8,179 | 6,457 | 7,517
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 | 3,506 | (11,063) | (4,848) | (8,174)
Operating Segments [Member] |  |  |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |  |
Total underwriting gain (loss), before federal income tax all segments | $ 80,653 | $ 79,689 | $ 200,001 | $ 238,759

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