Summary of Significant Accounting Policies - Fair Value (Details) - USD ($)	6 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2016	Jun. 30, 2016
Fair value hierarchy for the Company's financial assets measured at fair value
Interest rate (as a percent)			4.50%
Recurring basis
Fair value hierarchy for the Company's financial assets measured at fair value
Cash equivalents	$ 240,013,000	$ 240,013,000	$ 242,604,000
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	$ 240,013,000	$ 240,013,000	242,604,000
Recurring basis | 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			$ 2,800,000
Convertible Notes
Fair value hierarchy for the Company's financial assets measured at fair value
Interest rate (as a percent)	4.50%	4.50%	4.50%	4.50%
Principal amount of debt for conversion calculations	$ 1,000	$ 1,000			$ 1,000
Debt converted	$ 97,900,000	96,900,000
Shares issued with debt conversion (in shares)	26,160,187
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	$ 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	$ 3,800,000	$ 3,800,000