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Segment Information - Summary of Operating Results by Segment (Details) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2015 USD ($) State | Sep. 30, 2015 USD ($) | Jun. 30, 2015 USD ($) | Mar. 31, 2015 USD ($) | Dec. 31, 2014 USD ($) | Sep. 30, 2014 USD ($) | Jun. 30, 2014 USD ($) | Mar. 31, 2014 USD ($) | Dec. 31, 2015 USD ($) State Subsidiary | Dec. 31, 2014 USD ($) | Dec. 31, 2013 USD ($)
Segment Reporting [Abstract] |  |  |  |  |  |  |  |  |  |  |
Number of insurance companies | Subsidiary |  |  |  |  |  |  |  |  | 14 |  |
Number of states in which company operates | State | 13 |  |  |  |  |  |  |  | 13 |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net premiums earned | $ 760,094 | $ 745,520 | $ 731,546 | $ 720,737 | $ 709,368 | $ 705,237 | $ 697,889 | $ 683,701 | $ 2,957,897 | $ 2,796,195 | $ 2,698,187
Losses and loss adjustment expenses |  |  |  |  |  |  |  |  | 2,145,495 | 1,986,122 | 1,962,690
Underwriting expenses |  |  |  |  |  |  |  |  | 790,000 | 775,600 | 724,900
Underwriting gain |  |  |  |  |  |  |  |  | 22,400 | 34,500 | 10,600
Net investment income |  |  |  |  |  |  |  |  | 126,299 | 125,723 | 124,538
Net realized investment (losses) gains | (8,200) |  |  |  | (21,600) |  |  |  | (83,807) | 81,184 | (11,422)
Other |  |  |  |  |  |  |  |  | 8,911 | 8,671 | 9,738
Interest Expense |  |  |  |  |  |  |  |  | (3,168) | (2,637) | (1,260)
Pre-tax income | 23,930 | 12,267 | 4,511 | 29,859 | (28,161) | 37,120 | 136,436 | 102,030 | 70,567 | 247,425 | 132,096
Net income | $ 23,405 | $ 15,270 | $ 9,639 | $ 26,165 | $ (20,956) | $ 31,296 | $ 94,960 | $ 72,649 | 74,479 | 177,949 | 112,143
Property and Casualty Lines [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  | 2,906,600 | 2,737,300 | 2,638,400
Losses and loss adjustment expenses |  |  |  |  |  |  |  |  | 2,117,300 | 1,951,400 | 1,926,400
Underwriting expenses |  |  |  |  |  |  |  |  | 770,000 | 749,700 | 701,000
Underwriting gain |  |  |  |  |  |  |  |  | 19,300 | 36,200 | 11,000
Other Segments [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  | 51,300 | 58,900 | 59,800
Losses and loss adjustment expenses |  |  |  |  |  |  |  |  | 28,200 | 34,700 | 36,300
Underwriting expenses |  |  |  |  |  |  |  |  | 20,000 | 25,900 | 23,900
Underwriting gain |  |  |  |  |  |  |  |  | $ 3,100 | $ (1,700) | $ (400)

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