Derivative Instruments (Fair Values of Derivative Instruments in Consolidated Balance Sheets) (Details) - Not Designated as Hedging Instrument [Member] - Commodity Forward Contracts [Member] - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Derivatives, Fair Value [Line Items]
Fair value of derivative assets	$ 10,008	$ 5,553
Fair value of derivative liabilities	7,721	21,762
Accounts receivable, net [Member]
Derivatives, Fair Value [Line Items]
Fair value of derivative assets	4,491	3,465
Deferred charges and other assets, net [Member]
Derivatives, Fair Value [Line Items]
Fair value of derivative assets	5,517	2,088
Accrued liabilities [Member]
Derivatives, Fair Value [Line Items]
Fair value of derivative liabilities	2,215	9,325
Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Fair value of derivative liabilities	$ 5,506	$ 12,437