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Acquisitions Acquisitions - Narrative (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Business Acquisition [Line Items]      
Payments to Acquire Businesses, Net of Cash Acquired $ 0 $ 0 $ 10,000
Series of Individually Immaterial Business Acquisitions [Member]      
Business Acquisition [Line Items]      
Payments to Acquire Businesses, Net of Cash Acquired     $ 10,000