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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%

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- Definition Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258 + Details
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- Definition Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3044-108585 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section 45 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=82887183&loc=d3e6676-107765 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.1) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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