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Segment Information - Summary of Operating Results by Segment (Details)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2018
USD ($)
State
Jun. 30, 2017
USD ($)
Jun. 30, 2018
USD ($)
segment
Subsidiary
State
Jun. 30, 2017
USD ($)
Segment Reporting [Abstract]        
Number of insurance companies | Subsidiary     14  
Number of states in which company operates | State 11   11  
Number of reportable segments | segment     1  
Segment Reporting Information [Line Items]        
Net premiums earned $ 833,959 $ 797,666 $ 1,642,043 $ 1,587,436
Losses and loss adjustment expenses 605,547 588,595 1,237,781 1,195,260
Underwriting expenses 202,500 191,400 408,800 398,900
Underwriting (loss) gain 26,000 17,700 (4,600) (6,800)
Net investment income 34,786 31,901 66,296 63,070
Net realized investment gains (losses) 14,290 21,156 (44,445) 45,616
Other 2,356 2,124 4,681 4,229
Interest expense (4,256) (4,229) (8,522) (6,682)
Pre-tax income 73,246 68,752 13,547 99,351
Net income 60,180 51,633 17,573 78,613
Property and Casualty Lines [Member]        
Segment Reporting Information [Line Items]        
Net premiums earned 826,200 789,000 1,626,400 1,569,700
Losses and loss adjustment expenses 601,500 584,200 1,229,900 1,186,000
Underwriting expenses 198,700 187,400 401,200 391,000
Underwriting (loss) gain 26,000 17,400 (4,700) (7,300)
Other Segments [Member]        
Segment Reporting Information [Line Items]        
Net premiums earned 7,800 8,700 15,600 17,700
Losses and loss adjustment expenses 4,000 4,400 7,900 9,300
Underwriting expenses 3,800 4,000 7,600 7,900
Underwriting (loss) gain $ 0 $ 300 $ 100 $ 500