Derivative Instruments (Schedule of Derivative Instruments in Statement of Financial Position) (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Gross amount not offset against collateral, net	$ (25,095)	$ (48,892)
Derivative liabilities (assets), gross amounts offset in the consolidated balance sheets	0	0
Derivative Asset (Liability), Net	(25,095)	(48,892)
Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	2,417	6,507
Derivative Asset, Fair Value, Gross Liability	(1,381)	(5,270)
Derivative Asset	1,036	1,237
Other Noncurrent Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	106	1,313
Derivative Asset, Fair Value, Gross Liability	(106)	(1,313)
Derivative Asset	0	0
Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	(8,837)	(27,871)
Derivative Liability, Fair Value, Gross Asset	1,381	5,270
Derivative Liability	7,456	22,601
Other Noncurrent Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	(18,781)	(28,841)
Derivative Liability, Fair Value, Gross Asset	106	1,313
Derivative Liability	18,675	27,528
Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset	1,036	1,237
Derivative Liability	$ (21,970)	$ (45,552)