Subsequent events - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands			3 Months Ended	12 Months Ended
	Mar. 27, 2019	Nov. 06, 2018	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Mar. 01, 2019
Squadron Credit Agreement
Subsequent Event [Line Items]
Debt instrument, frequency of periodic payment		29 equal monthly installments
Debt instrument, maturity date		Nov. 30, 2023
Squadron Credit Agreement | Term Loan
Subsequent Event [Line Items]
Secured debt agreement		$ 35,000		$ 35,000
Squadron Credit Agreement | Line of credit | LIBOR
Subsequent Event [Line Items]
Line of credit, expiration period		5 years
Interest rate description		LIBOR plus 8%
Common Stock
Subsequent Event [Line Items]
Number of preferred shares converted to common stock				14,986,000	4,964,000
Common Stock | Squadron Credit Agreement
Subsequent Event [Line Items]
Number of warrants issued (in shares)		845,000
Exercise price of warrants		$ 3.15
Warrants term		7 years
Series A Convertible Preferred Stock
Subsequent Event [Line Items]
Number of preferred shares converted				1,274	9,927
Number of preferred shares outstanding for being conversion				4,043
Series A Convertible Preferred Stock | Common Stock
Subsequent Event [Line Items]
Number of preferred shares converted to common stock				636,997	4,963,702
Number of preferred shares outstanding for being conversion to common stock				2,021,673
Subsequent Event | Squadron Credit Agreement
Subsequent Event [Line Items]
Debt instrument, frequency of periodic payment	29 equal monthly installments
Debt instrument, maturity date	Nov. 30, 2023
Subsequent Event | Squadron Credit Agreement | Term Loan
Subsequent Event [Line Items]
Secured debt agreement	$ 30,000
Subsequent Event | Squadron Credit Agreement | Line of credit
Subsequent Event [Line Items]
Line of credit	$ 0
Floor on interest rate	10.00%
Ceiling on interest rate	13.00%
Subsequent Event | Squadron Credit Agreement | Line of credit | LIBOR
Subsequent Event [Line Items]
Interest rate description	LIBOR plus 8%
Subsequent Event | Common Stock | Squadron Credit Agreement
Subsequent Event [Line Items]
Number of warrants issued (in shares)	4,800,000
Exercise price of warrants	$ 2.17
Warrants term	7 years
Subsequent Event | Series A Convertible Preferred Stock
Subsequent Event [Line Items]
Number of preferred shares converted			3,715
Number of preferred shares outstanding for being conversion						328
Subsequent Event | Series A Convertible Preferred Stock | Common Stock
Subsequent Event [Line Items]
Number of preferred shares converted to common stock			1,857,586
Number of preferred shares outstanding for being conversion to common stock						164,087