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Revenue Recognition (Volume Commitment Programs) (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Dec. 31, 2018
Disaggregation of Revenue [Line Items]          
Revenue $ 620,103 $ 580,752 $ 1,196,159 $ 1,118,408  
Volume commitment programs          
Disaggregation of Revenue [Line Items]          
Commitment contract assets $ 60,400   60,400   $ 40,900
Commitment contract assets reclassified to accounts receivable   $ 2,300 $ 5,000