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Fair Value Measurement and Fair Value of Financial Instruments - Liabilities Carried at Fair Value Measured on Recurring Basis (Q3) (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Dec. 31, 2016
Fair Value, Measurements, Recurring      
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]      
Interest rate swap agreement, current liability $ 87 $ 1,163 $ 3,888
Fair Value, Measurements, Recurring | Level 1      
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]      
Liabilities at fair value 0 0 0
Fair Value, Measurements, Recurring | Level 2      
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]      
Liabilities at fair value 87 1,163 3,888
Fair Value, Measurements, Recurring | Level 3      
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]      
Liabilities at fair value 0 0 0
Interest rate swap agreements      
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]      
Interest rate swap agreement, current liability 87 1,163 3,888
Liabilities at fair value 87 1,163 3,888
Interest rate swap agreements | Fair Value, Measurements, Recurring      
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]      
Interest rate swap agreement, current liability 87 1,163 3,888
Interest rate swap agreements | Fair Value, Measurements, Recurring | Level 1      
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]      
Liabilities at fair value 0 0 0
Interest rate swap agreements | Fair Value, Measurements, Recurring | Level 2      
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]      
Liabilities at fair value 87 1,163 3,888
Interest rate swap agreements | Fair Value, Measurements, Recurring | Level 3      
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]      
Liabilities at fair value $ 0 $ 0 $ 0