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Fair Value Measurements - Narrative (Details) - USD ($) | 3 Months Ended |  |  |
--- | --- | --- | --- | ---
Mar. 31, 2018 | Jun. 30, 2018 | Dec. 31, 2017 | Aug. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Repayments of related party debt | $ 1,000,000 |  |  |
Convertible Notes | 2019 Convertible Senior Notes |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Aggregate principal amount outstanding |  | $ 223,215,000 | $ 245,000,000 | $ 245,000,000.0

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