Derivative Instruments and Hedging Activities (Schedule of Derivative Instruments in Statement of Financial Position, Fair Value) (Details) - Not Designated as Hedging Instrument - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Offsetting Assets and Liabilities [Line Items]
Derivative asset	$ 4,210	$ 406
Derivative liability	28,854	29,028
Current assets: Fair value of derivatives
Offsetting Assets and Liabilities [Line Items]
Derivative asset, fair value, gross asset	10,638	406
Derivative asset, fair value, gross liability	(6,428)	0
Derivative asset	4,210	406
Current liabilities: Fair value of derivatives
Offsetting Assets and Liabilities [Line Items]
Derivative liability, fair value, gross liability	(32,340)	(27,744)
Derivative liability, fair value, gross asset	6,428	0
Derivative liability	25,912	27,744
Long-term liabilities: Fair value of derivatives
Offsetting Assets and Liabilities [Line Items]
Derivative liability, fair value, gross liability	(2,942)	(1,284)
Derivative liability, fair value, gross asset	0	0
Derivative liability	$ 2,942	$ 1,284