Fair Value Measurements (Derivative Instruments Change in Fair Value of Level 3) (Details) - Derivative Commodity Instruments - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance at beginning of period		$ 24,436	$ 18,289	$ 89,019
Realized gains		13,145	22,208	55,210
Unrealized gains (losses) relating to instruments held at the reporting date	[1]	(40,495)	2,981	(71,367)
Settlements during period		(13,145)	(19,042)	(54,573)
Balance at end of period		(16,059)	24,436	18,289
Mark-to-market gains (losses)		$ (16,100)	$ 20,200	$ (7,600)

[1] Includes $16.1 million in mark-to-market losses, $20.2 million in mark-to-market gains and $7.6 million in mark-to-market losses for the years ended December 31, 2015, 2014 and 2013, respectively.