Convertible preferred stock (Details) - Series A Preferred stock $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($) shares	Apr. 11, 2016 shares
Convertible preferred stock
Number of Series A Preferred Stock converted			708,352
Number of common stock that Series A Preferred Stock converted into			2,162,907
Conversion ratio of common stock per share of Series A Preferred Stock	3.05	3.05	3.05
Shares designated (and issued)	1,535,398	2,243,750
Shares undesignated (and unissued)	1,756,250	1,756,250
Number of common shares into which each issued share to be converted	3.05	3.05	3.05
Total number of common shares into which issued shares to be converted	4,688,237	6,851,144
Aggregate liquidation preference | $	$ 6.1
Amount of purchase price to be received by holders in the event of acquisition | $	$ 6.1
Minimum
Convertible preferred stock
Percentage of preferred shareholders voting right required for automatic conversion		0.66
Valuation of the entity immediately prior to closing of initial public offering of common stock for automatic conversion | $		$ 50.0
Net proceeds of the offering for automatic conversion | $		$ 25.0