Common Stock Warrants (Details) - Class B common stock - $ / shares	Sep. 30, 2019	Dec. 31, 2018
Preferred stock warrants
Class of Warrant or Right [Line Items]
Warrants outstanding (in shares)	481,181	481,182
Warrant exercise price (in dollars per share)	$ 27.78	$ 27.78
Common stock warrants
Class of Warrant or Right [Line Items]
Warrants outstanding (in shares)	12,940	312,939
Warrant exercise price (in dollars per share)	$ 38.64	$ 38.64