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DERIVATIVE FINANCIAL INSTRUMENTS AND HEDGING - Narrative (FY) (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2018
Sep. 30, 2018
Dec. 31, 2017
Dec. 31, 2016
Derivative [Line Items]        
Net gain $ 4,700      
Accumulated other comprehensive income loss     $ (4,015) $ 2,957
Interest rate swap agreements        
Derivative [Line Items]        
Interest rate swap agreement, current liability 87 $ 87 $ 1,163 $ 3,888
Not Designated as Hedging Instrument | Foreign Exchange Forward        
Derivative [Line Items]        
Derivative term (in years)   1 year 1 year  
Net gain     $ 2,100  
Cash Flow Hedging | Designated as Hedging Instrument | Foreign Exchange Forward        
Derivative [Line Items]        
Derivative term (in years)     2 years  
Amount reclassified into income     $ 400  
Fair Value Hedging | Designated as Hedging Instrument | Interest rate swap agreements        
Derivative [Line Items]        
Interest rate swap agreement, current liability 100 $ 100 1,200  
Accumulated other comprehensive income loss $ 100 $ 100 $ 700