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Segment Information - Summary of Operating Results by Segment (Details) $ in Thousands | 3 Months Ended
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Mar. 31, 2017 USD ($) Subsidiary State | Mar. 31, 2016 USD ($)
Segment Reporting [Abstract] |  |
Number of insurance companies | Subsidiary | 14 |
Number of states in which company operates | State | 11 |
Segment Reporting Information [Line Items] |  |
Net premiums earned | $ 789,800 | $ 767,100
Losses and loss adjustment expenses | 606,665 | 594,082
Underwriting expenses | 207,800 | 202,800
Underwriting gain | (24,700) | (29,800)
Net investment income | 31,169 | 29,655
Net realized investment gains | 24,460 | 25,057
Other | 2,105 | 2,123
Interest Expense | (2,453) | (950)
Pre-tax income | 30,599 | 26,034
Net income | 26,980 | 23,323
Property and Casualty Lines [Member] |  |
Segment Reporting Information [Line Items] |  |
Net premiums earned | 780,800 | 755,800
Losses and loss adjustment expenses | 601,800 | 588,000
Underwriting expenses | 203,800 | 197,900
Underwriting gain | (24,800) | (30,100)
Other Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Net premiums earned | 9,000 | 11,300
Losses and loss adjustment expenses | 4,900 | 6,100
Underwriting expenses | 4,000 | 4,900
Underwriting gain | $ 100 | $ 300

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