Income taxes: Loss carry-forwards (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($)
Net operating loss carry-forwards
Net operating loss carry-forwards	$ 1,000,000
Transition tax	2,296
United States
Net operating loss carry-forwards
Net operating loss carry-forwards	150,900
Net operating loss carry-forwards not available for use	60,100
Europe
Net operating loss carry-forwards
Net operating loss carry-forwards	868,600
Net operating loss carry-forwards not subject to expiration	722,900
Net operating loss carry-forwards subject to expiration	145,700
Mexico
Net operating loss carry-forwards
Net operating loss carry-forwards	5,300
Asian
Net operating loss carry-forwards
Net operating loss carry-forwards	2,100
Canada
Net operating loss carry-forwards
Net operating loss carry-forwards	1,000
Other than United States
Net operating loss carry-forwards
Net operating loss carry-forwards	$ 877,000