Derivatives (Details 3) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Not designated as hedging | Cost of sales
Derivative Instruments, Gain (Loss) [Line Items]
Gains (Losses) on commodity derivatives included in cost of sales and merchandising revenues	$ 5,417	$ (15,012)	$ 62,541