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Segment Information - Summary of Operating Results by Segment (Details) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2017 USD ($) State | Sep. 30, 2017 USD ($) | Jun. 30, 2017 USD ($) | Mar. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Sep. 30, 2016 USD ($) | Jun. 30, 2016 USD ($) | Mar. 31, 2016 USD ($) | Dec. 31, 2017 USD ($) State Subsidiary | Dec. 31, 2016 USD ($) | Dec. 31, 2015 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 | $ 806,796 | $ 801,205 | $ 797,666 | $ 789,770 | $ 794,517 | $ 790,850 | $ 779,321 | $ 767,085 | $ 3,195,437 | $ 3,131,773 | $ 2,957,897
Losses and loss adjustment expenses |  |  |  |  |  |  |  |  | 2,444,884 | 2,355,138 | 2,145,495
Underwriting expenses |  |  |  |  |  |  |  |  | 788,700 | 797,900 | 790,000
Underwriting (loss) gain |  |  |  |  |  |  |  |  | (38,200) | (21,200) | 22,400
Net investment income |  |  |  |  |  |  |  |  | 124,930 | 121,871 | 126,299
Net realized investment gains (losses) |  |  |  |  |  |  |  |  | 83,650 | (34,255) | (83,807)
Other income |  |  |  |  |  |  |  |  | 11,945 | 8,294 | 8,911
Interest expense |  |  |  |  |  |  |  |  | (15,168) | (3,962) | (3,168)
Pre-tax income | 9,487 | 58,247 | 68,752 | 30,599 | (51,270) | 31,625 | 64,335 | 26,034 | 167,085 | 70,724 | 70,567
Net income | $ 19,779 | $ 46,485 | $ 51,633 | $ 26,980 | $ (26,082) | $ 26,930 | $ 48,873 | $ 23,323 | 144,877 | 73,044 | 74,479
Property and Casualty Lines [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  | 3,160,900 | 3,089,900 | 2,906,600
Losses and loss adjustment expenses |  |  |  |  |  |  |  |  | 2,427,800 | 2,333,200 | 2,117,300
Underwriting expenses |  |  |  |  |  |  |  |  | 773,100 | 780,400 | 770,000
Underwriting (loss) gain |  |  |  |  |  |  |  |  | (40,000) | (23,700) | 19,300
Other Segments [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  | 34,500 | 41,900 | 51,300
Losses and loss adjustment expenses |  |  |  |  |  |  |  |  | 17,100 | 21,900 | 28,200
Underwriting expenses |  |  |  |  |  |  |  |  | 15,600 | 17,500 | 20,000
Underwriting (loss) gain |  |  |  |  |  |  |  |  | $ 1,800 | $ 2,500 | $ 3,100

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