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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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Payments to contingent value right holders and other contingency payments, investing activities

+ References

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- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28551-108399

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