Income taxes: Reconciliation of unrecognized tax benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income taxes:
Unrecognized Tax Benefits Income Tax Penalties and Interest and Tax Accrued	$ 800	$ 1,000
Reconciliation of the beginning and ending amount of unrecognized tax benefits (excluding interest and penalties)
Beginning balance of unrecognized tax benefits	866	1,000	$ 1,312
Change attributable to tax positions taken during a prior period		82	(51)
Decrease attributable to settlements with taxing authorities		(160)
Decrease attributable to lapses of statutes of limitation	(90)	(56)	(261)
Ending balance of unrecognized tax benefits	$ 776	$ 866	$ 1,000