Stockholders' equity (Details) - $ / shares	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Preferred stock
Preferred stock, shares authorized (in shares)	15,000,000	15,000,000
Preferred stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common stock
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common stock, shares authorized (in shares)	100,000,000	100,000,000
Common stock, voting rights	1
Senior Managment [Member]
Treasury Stock [Abstract]
Stock repurchased as part of stock swap (in shares)