Summary of Significant Accounting Policies - Fair Value (Details) - USD ($)	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016
Fair value hierarchy for the Company's financial assets measured at fair value
Interest rate (as a percent)	4.50%		4.50%
Recurring basis
Fair value hierarchy for the Company's financial assets measured at fair value
Cash equivalents	$ 174,109,000	$ 144,176,000
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	$ 174,109,000	144,176,000
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			$ 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	3,100,000	100,000,000
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	$ 5,500,000	$ 79,000,000