Fair Value Measurements - Common Stock Warrants (Details) - $ / shares	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
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 5 months	3 years 8 months
Fair value of common stock warrants (in usd per share)	$ 17.75	$ 14.13
Warrant
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Common stock warrants outstanding (in shares)	354,350	354,350