Property, Plant and Mine Development (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Property, Plant and Equipment
Machinery and equipment	$ 1,226,715	$ 1,049,701
Accumulated depletion, depreciation and amortization	306,875	$ 266,388
Assets Held under Capital Leases
Property, Plant and Equipment
Machinery and equipment	4,300
Accumulated depletion, depreciation and amortization	$ 500