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Segment Information - Summary of Operating Results by Segment (Details) $ in Thousands | 3 Months Ended
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Mar. 31, 2019 USD ($) segment Subsidiary State | Mar. 31, 2018 USD ($)
Segment Reporting [Abstract] |  |
Number of insurance companies | Subsidiary | 14 |
Number of states in which company operates | State | 11 |
Number of reportable segments | segment | 1 |
Segment Reporting Information [Line Items] |  |
Net premiums earned | $ 870,245 | $ 808,084
Losses and loss adjustment expenses | 630,416 | 632,234
Underwriting expenses | 215,900 | 206,400
Underwriting gain (loss) | 23,900 | (30,500)
Net investment income | 34,174 | 31,510
Net realized investment gains (losses) | 111,074 | (58,735)
Other | 2,250 | 2,325
Interest expense | (4,256) | (4,266)
Pre-tax income (loss) | 167,169 | (59,699)
Net income (loss) | 135,867 | (42,607)
Property and Casualty Lines [Member] |  |
Segment Reporting Information [Line Items] |  |
Net premiums earned | 863,100 | 800,300
Losses and loss adjustment expenses | 626,800 | 628,300
Underwriting expenses | 212,500 | 202,600
Underwriting gain (loss) | 23,800 | (30,600)
Other Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Net premiums earned | 7,100 | 7,800
Losses and loss adjustment expenses | 3,600 | 3,900
Underwriting expenses | 3,400 | 3,800
Underwriting gain (loss) | $ 100 | $ 100

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