Derivative Instruments (Effect of Derivative Instruments on Operations) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Derivative [Line Items]
Income / (Expense)	$ (1,629)	$ 2,224	$ 3,937
Total derivatives designated as hedging instruments
Derivative [Line Items]
Income / (Expense)	(1,520)	1,467	2,655
Total derivatives designated as hedging instruments | Cost of goods sold | Commodity contracts
Derivative [Line Items]
Income / (Expense)	(1,520)	1,467	2,655
Total derivatives not designated as hedging instruments
Derivative [Line Items]
Income / (Expense)	(109)	757	1,282
Total derivatives not designated as hedging instruments | Cost of goods sold | Commodity contracts
Derivative [Line Items]
Income / (Expense)	(257)	246	131
Total derivatives not designated as hedging instruments | Realized and unrealized gain on derivatives | Commodity contracts
Derivative [Line Items]
Income / (Expense)	148	588	1,307
Total derivatives not designated as hedging instruments | Interest expense | Interest rate swap agreements
Derivative [Line Items]
Income / (Expense)	$ 0	$ (77)	$ (156)