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Segment Information - Summary of Operating Results by Segment (Details) $ in Thousands | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2017 USD ($) State | Jun. 30, 2016 USD ($) | Jun. 30, 2017 USD ($) Subsidiary State | Jun. 30, 2016 USD ($)
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 | $ 797,700 | $ 779,300 | $ 1,587,400 | $ 1,546,400
Losses and loss adjustment expenses | 588,595 | 595,086 | 1,195,260 | 1,189,168
Underwriting expenses | 191,300 | 197,600 | 399,000 | 400,400
Underwriting gain | 17,800 | (13,400) | (6,900) | (43,200)
Net investment income | 31,901 | 31,414 | 63,070 | 61,069
Net realized investment gains | 21,156 | 45,381 | 45,616 | 70,438
Other | 2,124 | 1,887 | 4,229 | 4,010
Interest Expense | (4,229) | (960) | (6,682) | (1,910)
Pre-tax income | 68,752 | 64,335 | 99,351 | 90,369
Net income | 51,633 | 48,873 | 78,613 | 72,196
Property and Casualty Lines [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 789,000 | 768,600 | 1,569,700 | 1,524,400
Losses and loss adjustment expenses | 584,200 | 589,400 | 1,186,000 | 1,177,400
Underwriting expenses | 187,300 | 193,400 | 391,100 | 391,300
Underwriting gain | 17,500 | (14,200) | (7,400) | (44,300)
Other Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 8,700 | 10,700 | 17,700 | 22,000
Losses and loss adjustment expenses | 4,400 | 5,700 | 9,300 | 11,800
Underwriting expenses | 4,000 | 4,200 | 7,900 | 9,100
Underwriting gain | $ 300 | $ 800 | $ 500 | $ 1,100

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