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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