Estimated Fair Value of Financial Instruments - Convertible Senior Notes (Details) - Convertible Senior Notes - Level 2 - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Convertible Senior Notes	$ 7,068	$ 7,055
Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Convertible Senior Notes	$ 26,400	$ 20,100

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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