Annual report pursuant to Section 13 and 15(d)

Acquisitions - Acquisitions Prior to 2015 (Details)

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Acquisitions - Acquisitions Prior to 2015 (Details) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Business Acquisition [Line Items]      
Gain (loss) due to changes in contingent cash payments $ 684,000 $ (877,000) $ (3,268,000)
Acquisitions Prior to 2015      
Business Acquisition [Line Items]      
Deferred cash obligation 100,000 4,100,000  
Paid deferred cash obligations 4,000,000 3,400,000  
Liability for the estimated cash payment $ 0    
Paid contingent cash obligations   $ 0