Inventories inventories - narrative (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Inventory Disclosure - Narrative [Abstract]
Inventory Valuation Reserves	$ 13,069				$ 13,069		$ 223,833
Lower of cost or market inventory valuation adjustment	(106,926)		$ 83,982		$ (210,764)	$ 95,805
Cost of Sales Reduction	$ 25,300	$ 33,800		$ 37,900