Derivative Instruments (Details 2) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Asset and liability derivatives
Asset derivative	$ 66,408	$ 107,647
Liability derivative	58,116	29,651
Commodity derivatives
Asset and liability derivatives
Asset derivative	66,408	107,647
Liability derivative	58,116	29,651
Commodity derivatives designated as hedging instruments | Commodity derivatives
Asset and liability derivatives
Asset derivative	33,942	104,430
Liability derivative	47,713	27,618
Commodity derivatives not designated as hedging instruments | Commodity derivatives
Asset and liability derivatives
Asset derivative	32,466	3,217
Liability derivative	$ 10,403	$ 2,033