Stockholders' equity (Details)	12 Months Ended
	Dec. 31, 2016 item $ / shares shares	Dec. 31, 2015 $ / shares shares
Stockholders' equity
Temporary Equity, Shares Issued	0
Temporary Equity, Shares Outstanding	0	0
Common Stock
Number of votes per share | item	1
Common Stock, Par or Stated Value Per Share | $ / shares	$ 0.01	$ 0.01
Common Stock, Shares Authorized	200,000,000	200,000,000
Common Stock, Shares, Issued	22,501,640	22,387,797
Common Stock, Shares, Outstanding	22,501,640	22,387,797
Series A Redeemable Preferred Stock
Stockholders' equity
Shares authorized	5,000,000
Par value (in dollars per share) | $ / shares	$ 0.01