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Goodwill (Details) - USD ($)
$ in Thousands
9 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Goodwill [Roll Forward]    
Goodwill at the beginning of year $ 287,956 $ 275,487
Increase in goodwill related to acquisitions 16,777 12,469
Goodwill at the end of the period $ 304,733 $ 287,956