Fair Value Measurements - Common Stock Warrants (Details) - $ / shares	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Weighted average exercise price (in usd per share)	$ 0.09	$ 0.09
Term (years)	3 years 11 months	4 years 8 months
Fair value of common stock warrants (in usd per share)	$ 20.33	$ 19.21
Warrant
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Common stock warrants outstanding (in shares)	354,350	354,350