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Goodwill (Goodwill Rollforward) (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Goodwill [Line Items]      
Goodwill, Acquired During Period $ 0 $ 153,628 $ 0
Goodwill, Impairment Loss 0 (12,462) 0
Goodwill [Roll Forward]      
Beginning balance 826,476 685,310 685,310
Final purchase price adjustment 61,748    
Ending balance $ 888,224 $ 826,476 $ 685,310