Income taxes: Net operating loss carry-forwards (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($)
Net operating loss carry-forwards
Combined net operating loss carry-forwards	$ 1,200.0
United States
Net operating loss carry-forwards
Combined net operating loss carry-forwards	465.3
Net operating loss carry-forwards not available for use	286.1
Net operating loss carry-forwards available for use	87.5
Europe
Net operating loss carry-forwards
Combined net operating loss carry-forwards	732.7
Net operating loss carry-forwards not subject to expiration	606.5
Net operating loss carry-forwards subject to expiration	126.2
Mexico
Net operating loss carry-forwards
Combined net operating loss carry-forwards	1.1
Asian
Net operating loss carry-forwards
Combined net operating loss carry-forwards	1.1
Other than United States
Net operating loss carry-forwards
Combined net operating loss carry-forwards	$ 734.9