Derivative Instruments and Hedging Activities (Schedule of Derivative Instruments in Statement of Financial Position, Fair Value) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Offsetting Assets and Liabilities [Line Items]
Derivative asset	$ 2,299	$ 0
Not Designated as Hedging Instrument
Offsetting Assets and Liabilities [Line Items]
Derivative asset	13,868	406
Derivative liability	(47,084)	(29,028)
Not Designated as Hedging Instrument \| Current assets: Fair value of derivatives
Offsetting Assets and Liabilities [Line Items]
Derivative asset, fair value, gross asset	33,530	406
Derivative asset, fair value, gross liability	(21,961)	0
Derivative asset	11,569	406
Not Designated as Hedging Instrument \| Long-term assets: Fair value of derivatives
Offsetting Assets and Liabilities [Line Items]
Derivative asset, fair value, gross asset	7,536
Derivative asset, fair value, gross liability	(5,237)
Derivative asset	2,299
Not Designated as Hedging Instrument \| Current liabilities: Fair value of derivatives
Offsetting Assets and Liabilities [Line Items]
Derivative liability, fair value, gross liability	(57,909)	(27,744)
Derivative liability, fair value, gross asset	21,961	0
Derivative liability	(35,948)	(27,744)
Not Designated as Hedging Instrument \| Long-term liabilities: Fair value of derivatives
Offsetting Assets and Liabilities [Line Items]
Derivative liability, fair value, gross liability	(16,373)	(1,284)
Derivative liability, fair value, gross asset	5,237	0
Derivative liability	$ (11,136)	$ (1,284)

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