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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, 2020 | Mar. 31, 2019
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |
Investment income | $ 11,301 | $ 64,069
Interest expense | (7,601) | (11,526)
Corporate expenses | (9,060) | (12,410)
Income before federal income tax | 15,997 | 73,694
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 | 17,126 | 25,815
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 | 387 | 3,167
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 | 3,844 | 4,579
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 | $ 32,658 | $ 97,630

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