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Segment Information - Summary of Operating Results by Segment (Details) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2018 USD ($) State | Sep. 30, 2017 USD ($) | Sep. 30, 2018 USD ($) Subsidiary segment State | Sep. 30, 2017 USD ($)
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 | $ 858,135 | $ 801,205 | $ 2,500,178 | $ 2,388,641
Losses and loss adjustment expenses | 614,069 | 595,290 | 1,851,850 | 1,790,550
Underwriting expenses | 206,100 | 200,600 | 614,900 | 599,600
Underwriting gain (loss) | 37,900 | 5,300 | 33,400 | (1,600)
Net investment income | 38,159 | 30,988 | 104,455 | 94,058
Net realized investment (losses) gains | (3,910) | 20,718 | (48,355) | 66,334
Other | 2,427 | 5,446 | 7,108 | 9,675
Interest expense | (4,257) | (4,191) | (12,779) | (10,873)
Pre-tax income | 70,286 | 58,247 | 83,833 | 157,598
Net income | 58,578 | 46,485 | 76,151 | 125,098
Property and Casualty Lines [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 850,500 | 792,700 | 2,476,900 | 2,362,400
Losses and loss adjustment expenses | 610,200 | 591,100 | 1,840,100 | 1,777,100
Underwriting expenses | 202,800 | 196,700 | 604,000 | 587,700
Underwriting gain (loss) | 37,500 | 4,900 | 32,800 | (2,400)
Other Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 7,600 | 8,500 | 23,300 | 26,200
Losses and loss adjustment expenses | 3,900 | 4,200 | 11,800 | 13,500
Underwriting expenses | 3,300 | 3,900 | 10,900 | 11,900
Underwriting gain (loss) | $ 400 | $ 400 | $ 600 | $ 800

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