Summary of Significant Accounting Policies - Fair Value of Financial Instruments (Details)	9 Months Ended
	Sep. 30, 2019 USD ($) item	Dec. 31, 2018 USD ($)	Dec. 31, 2017	Dec. 31, 2016 USD ($)
Fair value hierarchy for the Company's financial assets measured at fair value
Interest rate (as a percent)	4.50%	4.50%
Number of trades | item	0
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
Significant Other Observable Inputs (Level 2) | Convertible Notes | 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
Significant Other Observable Inputs (Level 2) | Convertible Notes | Estimated fair value
Fair value hierarchy for the Company's financial assets measured at fair value
Convertible debt fair value		2,800,000
Recurring basis
Fair value hierarchy for the Company's financial assets measured at fair value
Cash equivalents	187,759,000	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	187,759,000	242,604,000
Recurring basis | Significant Other Observable Inputs (Level 2)
Fair value hierarchy for the Company's financial assets measured at fair value
Cash equivalents	0	0
Recurring basis | Significant Unobservable Inputs (Level 3)
Fair value hierarchy for the Company's financial assets measured at fair value
Cash equivalents	$ 0	$ 0