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Segment Information - Summary of Operating Results by Segment (Details) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2019 USD ($) State | Sep. 30, 2019 USD ($) | Jun. 30, 2019 USD ($) | Mar. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Sep. 30, 2018 USD ($) | Jun. 30, 2018 USD ($) | Mar. 31, 2018 USD ($) | Dec. 31, 2019 USD ($) State Subsidiary segment | Dec. 31, 2018 USD ($) | Dec. 31, 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 | $ 925,384 | $ 915,012 | $ 888,776 | $ 870,245 | $ 868,233 | $ 858,135 | $ 833,959 | $ 808,084 | $ 3,599,418 | $ 3,368,411 | $ 3,195,437
Losses and loss adjustment expenses |  |  |  |  |  |  |  |  | 2,706,024 | 2,576,789 | 2,444,884
Underwriting expenses |  |  |  |  |  |  |  |  | 871,400 | 816,800 | 788,700
Underwriting gain (loss) |  |  |  |  |  |  |  |  | 22,000 | (25,200) | (38,200)
Net investment income |  |  |  |  |  |  |  |  | 141,263 | 135,838 | 124,930
Net realized investment gains (losses) |  |  |  |  |  |  |  |  | 222,793 | (133,520) | 83,650
Other income |  |  |  |  |  |  |  |  | 9,044 | 9,275 | 11,945
Interest expense |  |  |  |  |  |  |  |  | (17,035) | (17,036) | (15,168)
Income (loss) before income taxes | 28,465 | 80,840 | 101,595 | 167,169 | (114,448) | 70,286 | 73,246 | (59,699) | 378,069 | (30,615) | 167,085
Net income (loss) | $ 31,688 | $ 69,282 | $ 83,250 | $ 135,867 | $ (81,879) | $ 58,578 | $ 60,180 | $ (42,607) | 320,087 | (5,728) | 144,877
Property and Casualty Lines [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  | 3,571,000 | 3,337,900 | 3,160,900
Losses and loss adjustment expenses |  |  |  |  |  |  |  |  | 2,692,700 | 2,562,000 | 2,427,800
Underwriting expenses |  |  |  |  |  |  |  |  | 857,300 | 802,700 | 773,100
Underwriting gain (loss) |  |  |  |  |  |  |  |  | 21,000 | (26,800) | (40,000)
Other Segments [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  | 28,400 | 30,500 | 34,500
Losses and loss adjustment expenses |  |  |  |  |  |  |  |  | 13,300 | 14,800 | 17,100
Underwriting expenses |  |  |  |  |  |  |  |  | 14,100 | 14,100 | 15,600
Underwriting gain (loss) |  |  |  |  |  |  |  |  | $ 1,000 | $ 1,600 | $ 1,800

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