Summary of Significant Accounting Policies - Fair Value (Details)	6 Months Ended
	Jun. 30, 2018 USD ($) item	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Fair value hierarchy for the Company's financial assets measured at fair value
Number of trades | item	0
Recurring basis
Fair value hierarchy for the Company's financial assets measured at fair value
Cash equivalents	$ 323,388,000	$ 240,013,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair value hierarchy for the Company's financial assets measured at fair value
Cash equivalents	$ 323,388,000	$ 240,013,000
Convertible Notes
Fair value hierarchy for the Company's financial assets measured at fair value
Interest rate (as a percent)	4.50%	4.50%
Principal amount of debt for conversion calculations			$ 1,000
Convertible Notes | Significant Other Observable Inputs (Level 2) | Face Value
Fair value hierarchy for the Company's financial assets measured at fair value
Convertible debt fair value	$ 2,100,000	$ 2,100,000
Convertible Notes | Significant Other Observable Inputs (Level 2) | Estimated fair value
Fair value hierarchy for the Company's financial assets measured at fair value
Convertible debt fair value	$ 5,700,000	$ 3,800,000