Common stock and redeemable convertible preferred stock - Rights and Preferences of Preferred Stock (Details)			12 Months Ended
	Nov. 01, 2017 USD ($) installment $ / shares	Jul. 28, 2015 shares	Dec. 31, 2015 USD ($) $ / shares	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Mar. 31, 2015 USD ($)
Temporary equity related disclosures
Fair value of warrants				$ 1,789,000
Redeemable preferred stock value				90,149,000
Series C Warrants Issued with Series C Redeemable Preferred Stock Financing
Temporary equity related disclosures
Fair value adjustment of warrants (reduction) increase			$ 1,698,000
Series C Warrants Issued with Series C Redeemable Preferred Stock Financing | Common Stock | Cashless exercise of redeemable convertible preferred stock warrant into common stock
Temporary equity related disclosures
Shares issued on conversion | shares		78,926
All series of redeemable convertible preferred stock | Common Stock | Conversion of redeemable convertible preferred stock into common stock
Temporary equity related disclosures
Shares issued on conversion | shares		9,217,983
Series A preferred stock
Temporary equity related disclosures
Original issue price (in dollars per share) | $ / shares			$ 5.00
Redeemable preferred stock value				1,068,000
Series C preferred stock
Temporary equity related disclosures
Original issue price (in dollars per share) | $ / shares			$ 5.00
Accretion to redemption value			$ 607,000	87,000	$ 78,000
Redeemable preferred stock value				42,131,000
Series B-1 preferred stock
Temporary equity related disclosures
Dividend accrual rate (as a percent)			8.00%
Original issue price (in dollars per share) | $ / shares			$ 5.00
Accretion to redemption value			$ 370,000	679,000	756,000
Redeemable preferred stock value				32,391,000
Series B preferred stock
Temporary equity related disclosures
Original issue price (in dollars per share) | $ / shares			$ 5.00
Accretion to redemption value			$ 192,000	352,000	$ 394,000
Redeemable preferred stock value				14,559,000
Redeemable convertible preferred stock financing | Series C preferred stock
Temporary equity related disclosures
Redeemable preferred stock value				6,108,000		$ 10,916,000
Stock issuance legal fees				57,000		78,000
Redeemable convertible preferred stock financing | Series C preferred stock | Series C Warrants Issued with Series C Redeemable Preferred Stock Financing
Temporary equity related disclosures
Fair value of warrants				$ 1,335,000		$ 2,131,000
Forecast | Series C, Series B-1, and Series B preferred stock
Temporary equity related disclosures
Original issue price (in dollars per share) | $ / shares	$ 5.00
Number of installments for redemption | installment	3
Notice period for redemption	60 days
Redemption value (in dollars per share) | $ / shares	$ 5.00
Forecast | Series B-1 preferred stock
Temporary equity related disclosures
Minimum redemption value upon initial redemption date	$ 27,309,000
Forecast | Series B preferred stock
Temporary equity related disclosures
Minimum redemption value upon initial redemption date	15,565,000
Forecast Related To Issuances Through December 2014 | Series C preferred stock
Temporary equity related disclosures
Minimum redemption value upon initial redemption date	43,768,000
Forecast Related To Issuances Through IPO Date | Series C preferred stock
Temporary equity related disclosures
Minimum redemption value upon initial redemption date	$ 57,642,000