Derivative Instruments (Impact of Derivative Instruments on Consolidated Balance Sheet) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Commodity Forward Contracts [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 12,416	$ 14,986
Derivative Liability, Fair Value, Gross Liability	7,618	7,221
Accounts receivable, net [Member]
Derivative [Line Items]
Derivative Asset, Current	5,350	7,589
Derivative Asset, Fair Value, Gross Asset	7,335	7,727
Derivative Liability, Fair Value, Gross Liability	1,985	138
Accounts receivable, net [Member] | Subject to Master Netting [Member]
Derivative [Line Items]
Derivative Asset, Current	793	1,498
Derivative Asset, Fair Value, Gross Asset	2,778	1,636
Derivative Liability, Fair Value, Gross Liability	1,985	138
Accounts receivable, net [Member] | Not Subject to Master Netting [Member]
Derivative [Line Items]
Derivative Asset, Current	4,557	6,091
Derivative Asset, Fair Value, Gross Asset	4,557	6,091
Derivative Liability, Fair Value, Gross Liability	0	0
Deferred Charges and other assets, net [Member]
Derivative [Line Items]
Derivative Asset, Noncurrent	4,336	5,249
Derivative Asset, Fair Value, Gross Asset	4,336	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	4,336	5,249
Derivative Asset, Fair Value, Gross Asset	4,336	5,249
Derivative Liability, Fair Value, Gross Liability	0	0
Accrued Liabilities [Member]
Derivative [Line Items]
Derivative Liability, Current	704	1,349
Derivative Asset, Fair Value, Gross Asset	0	0
Derivative Liability, Fair Value, Gross Liability	704	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	704	1,349
Derivative Asset, Fair Value, Gross Asset	0	0
Derivative Liability, Fair Value, Gross Liability	704	1,349
Other Liabilities [Member]
Derivative [Line Items]
Derivative Liability, Noncurrent	4,184	3,724
Derivative Asset, Fair Value, Gross Asset	745	2,010
Derivative Liability, Fair Value, Gross Liability	4,929	5,734
Other Liabilities [Member] | Subject to Master Netting [Member]
Derivative [Line Items]
Derivative Liability, Noncurrent	1,410	436
Derivative Asset, Fair Value, Gross Asset	745	2,010
Derivative Liability, Fair Value, Gross Liability	2,155	2,446
Other Liabilities [Member] | Not Subject to Master Netting [Member]
Derivative [Line Items]
Derivative Liability, Noncurrent	2,774	3,288
Derivative Asset, Fair Value, Gross Asset	0	0
Derivative Liability, Fair Value, Gross Liability	$ 2,774	$ 3,288