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Goodwill (Changes in Goodwill) (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Goodwill [Roll Forward]    
Goodwill at the beginning of year $ 275,487 $ 264,416
Increase in goodwill related to acquisitions 12,469 11,832
Decrease in goodwill related to divestitures 0 (761)
Goodwill at the end of the year $ 287,956 $ 275,487