Leases (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating Leased Assets [Line Items]
Rent expense, operating lease	$ 48,052	$ 77,879	$ 87,903
Property, Plant and Equipment	4,838,171	4,676,353
Accumulated depreciation	2,731,643	2,554,056
Mining Equipment
Operating Leased Assets [Line Items]
Property, Plant and Equipment	10,097	16,672
Accumulated depreciation	$ 10,097	$ 13,337