Inventories inventories - narrative (Details) - USD ($) $ in Thousands	3 Months Ended				9 Months Ended
	Sep. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Inventory Disclosure - Narrative [Abstract]
Inventory Valuation Reserves	$ 30,906				$ 30,906		$ 223,833
Lower of cost or market inventory valuation adjustment	17,837		$ (111,128)		$ (192,927)	$ (15,323)
Cost of Sales Reduction	$ 25,300	$ 33,800		$ 37,900