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Cash and Cash Equivalents and Fair Value of Financial Instruments - Narrative (Details) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Mar. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]          
Cash and cash equivalents $ 77,244 $ 67,192 $ 43,773 $ 32,424  
Fair value of the interest rate swap agreements $ 136 $ 562      
Interest Rate Swap          
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]          
Derivative, notional amount         $ 100,000
Derivative, fixed interest rate         0.80%
London Interbank Offered Rate (LIBOR) | Interest Rate Swap          
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]          
Derivative, basis spread on variable rate         0.00%