Derivative Instruments (Details 2) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Asset and liability derivatives
Asset derivative	$ 79,821	$ 107,647
Liability derivative	59,333	29,651
Commodity contracts
Asset and liability derivatives
Asset derivative	79,821	107,647
Liability derivative	59,333	29,651
Commodity derivatives designated as hedging instruments | Commodity contracts
Asset and liability derivatives
Asset derivative	74,953	104,430
Liability derivative	57,426	27,618
Commodity derivatives not designated as hedging instruments | Commodity contracts
Asset and liability derivatives
Asset derivative	4,868	3,217
Liability derivative	$ 1,907	$ 2,033