Derivative Commodity Instruments (Gain (Loss) on Derivative Instruments) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Derivative [Line Items]
Gain (loss) on derivative instruments, net	$ 115,293	$ 335,019	$ (50,024)
Commodity contracts
Derivative [Line Items]
Open non-cash mark-to-market gains (losses) on derivative instruments	(281,752)	315,445	(47,832)
Closed gains (losses) on derivative instruments	397,045	19,574	(2,192)
Gain (loss) on derivative instruments, net	$ 115,293	$ 335,019	$ (50,024)