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Segment Information - Summary of Operating Results by Segment (Details) $ in Thousands | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2018 USD ($) State | Jun. 30, 2017 USD ($) | Jun. 30, 2018 USD ($) segment Subsidiary State | Jun. 30, 2017 USD ($)
Segment Reporting [Abstract] |  |  |  |
Number of insurance companies | Subsidiary |  |  | 14 |
Number of states in which company operates | State | 11 |  | 11 |
Number of reportable segments | segment |  |  | 1 |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | $ 833,959 | $ 797,666 | $ 1,642,043 | $ 1,587,436
Losses and loss adjustment expenses | 605,547 | 588,595 | 1,237,781 | 1,195,260
Underwriting expenses | 202,500 | 191,400 | 408,800 | 398,900
Underwriting (loss) gain | 26,000 | 17,700 | (4,600) | (6,800)
Net investment income | 34,786 | 31,901 | 66,296 | 63,070
Net realized investment gains (losses) | 14,290 | 21,156 | (44,445) | 45,616
Other | 2,356 | 2,124 | 4,681 | 4,229
Interest expense | (4,256) | (4,229) | (8,522) | (6,682)
Pre-tax income | 73,246 | 68,752 | 13,547 | 99,351
Net income | 60,180 | 51,633 | 17,573 | 78,613
Property and Casualty Lines [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 826,200 | 789,000 | 1,626,400 | 1,569,700
Losses and loss adjustment expenses | 601,500 | 584,200 | 1,229,900 | 1,186,000
Underwriting expenses | 198,700 | 187,400 | 401,200 | 391,000
Underwriting (loss) gain | 26,000 | 17,400 | (4,700) | (7,300)
Other Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 7,800 | 8,700 | 15,600 | 17,700
Losses and loss adjustment expenses | 4,000 | 4,400 | 7,900 | 9,300
Underwriting expenses | 3,800 | 4,000 | 7,600 | 7,900
Underwriting (loss) gain | $ 0 | $ 300 | $ 100 | $ 500

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