Fair Value Measurements--Changes in Fair Value of Derivatives Classified as Level 3 (Details) (Recurring Fair Value Measurements [Member], Significant unobservable inputs (Level 3) [Member], Derivative [Member], USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Recurring Fair Value Measurements [Member] | Significant unobservable inputs (Level 3) [Member] | Derivative [Member]
Level 3 Rollforward
Beginning balance	$ 3,622	$ 26,231
Realized and unrealized gains (losses) included in non-hedge derivative (losses) gains	180,539	(8,725)
Purchases	20,609	664
Settlements received	(9,603)	(14,548)
Ending balance	195,167	3,622
Gains relating to instruments still held at the reporting date included in non-hedge derivative gains for the period	$ 169,442	$ 3,177