Income taxes: Reconciliation of unrecognized tax benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reconciliation of the beginning and ending amount of unrecognized tax benefits (excluding interest and penalties)
Beginning balance of unrecognized tax benefits	$ 776	$ 866	$ 1,000
Change attributable to tax positions taken during a prior period			82
Decrease attributable to settlements with taxing authorities	$ (776)		(160)
Decrease attributable to lapses of statutes of limitation		(90)	(56)
Ending balance of unrecognized tax benefits		$ 776	$ 866