Stockholders' equity: Authorized shares (Details)	12 Months Ended
	Dec. 31, 2017 Vote / shares $ / shares shares	Dec. 31, 2016 $ / shares shares
Stockholders' equity:
Common stock, shares authorized | shares	75,000,000	75,000,000
Common stock, par value (in dollars per share) | $ / shares	$ 0.001	$ 0.001
Preferred stock, authorized but unissued shares (in shares) | shares	10,000
Preferred stock, par value (in dollars per share) | $ / shares	$ 0.001
Voting rights per common share | Vote / shares	1