Derivative Instruments (Impact of Derivative Instruments on Consolidated Balance Sheet) (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Commodity Forward Contracts [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 6,877	$ 14,986
Derivative Liability, Fair Value, Gross Liability	(7,670)	(7,221)
Accounts receivable, net [Member]
Derivative [Line Items]
Derivative Asset, Current	3,124	7,589
Derivative Asset, Fair Value, Gross Asset	3,653	7,727
Derivative Liability, Fair Value, Gross Liability	(529)	(138)
Accounts receivable, net [Member] | Subject to Master Netting [Member]
Derivative [Line Items]
Derivative Asset, Current	520	1,498
Derivative Asset, Fair Value, Gross Asset	1,049	1,636
Derivative Liability, Fair Value, Gross Liability	(529)	(138)
Accounts receivable, net [Member] | Not Subject to Master Netting [Member]
Derivative [Line Items]
Derivative Asset, Current	2,604	6,091
Derivative Asset, Fair Value, Gross Asset	2,604	6,091
Derivative Liability, Fair Value, Gross Liability	0	0
Deferred Charges and other assets, net [Member]
Derivative [Line Items]
Derivative Asset, Noncurrent	2,246	5,249
Derivative Asset, Fair Value, Gross Asset	2,246	5,249
Derivative Liability, Fair Value, Gross Liability	0	0
Deferred Charges and other assets, net [Member] | Subject to Master Netting [Member]
Derivative [Line Items]
Derivative Asset, Noncurrent	0	0
Derivative Asset, Fair Value, Gross Asset	0	0
Derivative Liability, Fair Value, Gross Liability	0	0
Deferred Charges and other assets, net [Member] | Not Subject to Master Netting [Member]
Derivative [Line Items]
Derivative Asset, Noncurrent	2,246	5,249
Derivative Asset, Fair Value, Gross Asset	2,246	5,249
Derivative Liability, Fair Value, Gross Liability	0	0
Accrued Liabilities [Member]
Derivative [Line Items]
Derivative Liability, Current	(1,048)	(1,349)
Derivative Asset, Fair Value, Gross Asset	0	0
Derivative Liability, Fair Value, Gross Liability	(1,048)	(1,349)
Accrued Liabilities [Member] | Subject to Master Netting [Member]
Derivative [Line Items]
Derivative Liability, Current	0	0
Derivative Asset, Fair Value, Gross Asset	0	0
Derivative Liability, Fair Value, Gross Liability	0	0
Accrued Liabilities [Member] | Not Subject to Master Netting [Member]
Derivative [Line Items]
Derivative Liability, Current	(1,048)	(1,349)
Derivative Asset, Fair Value, Gross Asset	0	0
Derivative Liability, Fair Value, Gross Liability	(1,048)	(1,349)
Other Liabilities [Member]
Derivative [Line Items]
Derivative Liability, Noncurrent	(5,115)	(3,724)
Derivative Asset, Fair Value, Gross Asset	978	2,010
Derivative Liability, Fair Value, Gross Liability	(6,093)	(5,734)
Other Liabilities [Member] | Subject to Master Netting [Member]
Derivative [Line Items]
Derivative Liability, Noncurrent	(1,629)	(436)
Derivative Asset, Fair Value, Gross Asset	978	2,010
Derivative Liability, Fair Value, Gross Liability	(2,607)	(2,446)
Other Liabilities [Member] | Not Subject to Master Netting [Member]
Derivative [Line Items]
Derivative Liability, Noncurrent	(3,486)	(3,288)
Derivative Asset, Fair Value, Gross Asset	0	0
Derivative Liability, Fair Value, Gross Liability	$ (3,486)	$ (3,288)