Convertible preferred stock (Details) (USD $) In Millions, except Share data, unless otherwise specified			12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2013 Series A Preferred Stock	Dec. 31, 2013 Series A Preferred Stock Minimum
Convertible preferred stock
Shares designated (and issued)	2,243,750	2,243,750	2,243,750
Shares undesignated (and unissued)			1,756,250
Number of common shares into which each issued share to be converted			3.05
Total number of common shares into which issued shares to be converted			6,851,144
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
Aggregate liquidation preference			9.0
Amount of purchase price to be received by holders in the event of acquisition			$ 9.0