Cash and Cash Equivalents and Fair Value of Financial Instruments - Narrative (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 87,482	$ 67,192
Fair value of the interest rate swap agreements		$ 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%