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Segment Information - Summary of Operating Results by Segment (Details) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2016 USD ($) State | Sep. 30, 2016 USD ($) | Jun. 30, 2016 USD ($) | Mar. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Sep. 30, 2015 USD ($) | Jun. 30, 2015 USD ($) | Mar. 31, 2015 USD ($) | Dec. 31, 2016 USD ($) State Subsidiary | Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($)
Segment Reporting [Abstract] |  |  |  |  |  |  |  |  |  |  |
Number of insurance companies | Subsidiary |  |  |  |  |  |  |  |  | 14 |  |
Number of states in which company operates | State | 11 |  |  |  |  |  |  |  | 11 |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net premiums earned | $ 794,517 | $ 790,850 | $ 779,321 | $ 767,085 | $ 760,094 | $ 745,520 | $ 731,546 | $ 720,737 | $ 3,131,773 | $ 2,957,897 | $ 2,796,195
Losses and loss adjustment expenses |  |  |  |  |  |  |  |  | 2,355,138 | 2,145,495 | 1,986,122
Underwriting expenses |  |  |  |  |  |  |  |  | 797,900 | 790,000 | 775,600
Underwriting (loss) gain |  |  |  |  |  |  |  |  | (21,200) | 22,400 | 34,500
Net investment income |  |  |  |  |  |  |  |  | 121,871 | 126,299 | 125,723
Net realized investment (losses) gains |  |  |  |  | (8,200) |  |  |  | (34,255) | (83,807) | 81,184
Other |  |  |  |  |  |  |  |  | 8,294 | 8,911 | 8,671
Interest Expense |  |  |  |  |  |  |  |  | (3,962) | (3,168) | (2,637)
Pre-tax income | (51,270) | 31,625 | 64,335 | 26,034 | 23,930 | 12,267 | 4,511 | 29,859 | 70,724 | 70,567 | 247,425
Net income | $ (26,082) | $ 26,930 | $ 48,873 | $ 23,323 | $ 23,405 | $ 15,270 | $ 9,639 | $ 26,165 | 73,044 | 74,479 | 177,949
Property and Casualty Lines [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  | 3,089,900 | 2,906,600 | 2,737,300
Losses and loss adjustment expenses |  |  |  |  |  |  |  |  | 2,333,200 | 2,117,300 | 1,951,400
Underwriting expenses |  |  |  |  |  |  |  |  | 780,400 | 770,000 | 749,700
Underwriting (loss) gain |  |  |  |  |  |  |  |  | (23,700) | 19,300 | 36,200
Other Segments [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  | 41,900 | 51,300 | 58,900
Losses and loss adjustment expenses |  |  |  |  |  |  |  |  | 21,900 | 28,200 | 34,700
Underwriting expenses |  |  |  |  |  |  |  |  | 17,500 | 20,000 | 25,900
Underwriting (loss) gain |  |  |  |  |  |  |  |  | $ 2,500 | $ 3,100 | $ (1,700)

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