Derivatives (Details 2) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Net derivative asset position, Fair value of derivatives	$ 31,055	$ 61,247
Commodity derivative liabilities	0	0
Commodity derivative assets - current	71,319	103,105
Commodity derivative assets - noncurrent	246	1,906
Commodity derivative liabilities - current	(63,954)	(33,277)
Commodity derivative liabilities - noncurrent	(6,644)	(1,134)
Commodity Contract
Derivatives, Fair Value [Line Items]
Net derivative asset position, Fair value of derivatives	70,870	144,541
Commodity derivative liabilities	(85,383)	(60,169)
Cash collateral	15,480	(13,772)
Total	967	70,600
Commodity Contract | Commodity derivative assets - current
Derivatives, Fair Value [Line Items]
Net derivative asset position, Fair value of derivatives	69,289	137,119
Commodity derivative liabilities	(13,450)	(20,242)
Cash collateral	15,480	(13,772)
Commodity derivative assets - current	71,319	103,105
Commodity Contract | Commodity derivative assets - noncurrent
Derivatives, Fair Value [Line Items]
Net derivative asset position, Fair value of derivatives	246	2,059
Commodity derivative liabilities	0	(153)
Cash collateral	0	0
Commodity derivative assets - noncurrent	246	1,906
Commodity Contract | Commodity derivative liabilities - current
Derivatives, Fair Value [Line Items]
Net derivative asset position, Fair value of derivatives	1,286	5,233
Commodity derivative liabilities	(65,240)	(38,510)
Cash collateral	0	0
Commodity derivative liabilities - current	(63,954)	(33,277)
Commodity Contract | Commodity derivative liabilities - noncurrent
Derivatives, Fair Value [Line Items]
Net derivative asset position, Fair value of derivatives	49	130
Commodity derivative liabilities	(6,693)	(1,264)
Cash collateral	0	0
Commodity derivative liabilities - noncurrent	$ (6,644)	$ (1,134)