Inventories inventories - narrative (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Inventory Disclosure - Narrative [Abstract]
Inventory Valuation Reserves	$ 119,995		$ 223,833
Lower of cost or market inventory valuation adjustment	(103,838)	$ 11,823
Renewable Volume Obligation	$ 33,800		$ 37,900