Leases (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Leased Assets [Line Items]
Rent expense, operating lease	$ 77,879	$ 87,903	$ 83,423
Property, Plant and Equipment	4,676,353	4,593,395
Accumulated depreciation	2,554,056	2,413,125
Mining Equipment
Operating Leased Assets [Line Items]
Property, Plant and Equipment	16,672	26,005
Accumulated depreciation	$ 13,337	$ 15,603