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Segment Information (Reconciliation of Segment Results to Income Before Federal Income Tax (Details)) - USD ($) $ in Thousands | 3 Months Ended
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Mar. 31, 2019 | Mar. 31, 2018
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |
Investment income, before federal income tax | $ 64,069 | $ 32,682
Interest expense | (11,526) | (6,152)
Corporate expenses | (12,410) | (11,332)
Income before federal income tax | 73,694 | 19,931
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 | 25,815 | 6,804
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 | 3,167 | (1,506)
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 | 4,579 | (565)
Operating Segments [Member] |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |
Total income, before federal income tax all segments | $ 97,630 | $ 37,415

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