Derivatives (Details 1) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2013
Derivatives, Fair Value [Line Items]
Commodity derivative assets	$ (88,498)	$ 31,055	$ 35,403
Commodity derivative liabilities	0	0	0
Commodity derivative assets - current	119,330	71,319	158,079
Commodity derivative assets - noncurrent	1,365	246	813
Commodity derivative liabilities - current	(32,153)	(63,954)	(50,157)
Commodity derivative liabilities - noncurrent	(734)	(6,644)	(3,220)
Commodity
Derivatives, Fair Value [Line Items]
Commodity derivative assets	92,915	70,870	106,584
Commodity derivative liabilities	(147,898)	(85,383)	(74,102)
Cash collateral	142,791	15,480	73,033
Total	87,808	967	105,515
Commodity | Commodity derivative assets - current
Derivatives, Fair Value [Line Items]
Commodity derivative assets	86,512	69,289	101,833
Commodity derivative liabilities	(109,973)	(13,450)	(16,787)
Cash collateral	142,791	15,480	73,033
Commodity derivative assets - current	119,330	71,319	158,079
Commodity | Commodity derivative assets - noncurrent
Derivatives, Fair Value [Line Items]
Commodity derivative assets	1,543	246	834
Commodity derivative liabilities	(178)	0	(21)
Cash collateral	0	0	0
Commodity derivative assets - noncurrent	1,365	246	813
Commodity | Commodity derivative liabilities - current
Derivatives, Fair Value [Line Items]
Commodity derivative assets	4,552	1,286	3,898
Commodity derivative liabilities	(36,705)	(65,240)	(54,055)
Cash collateral	0	0	0
Commodity derivative liabilities - current	(32,153)	(63,954)	(50,157)
Commodity | Commodity derivative liabilities - noncurrent
Derivatives, Fair Value [Line Items]
Commodity derivative assets	308	49	19
Commodity derivative liabilities	(1,042)	(6,693)	(3,239)
Cash collateral	0	0	0
Commodity derivative liabilities - noncurrent	$ (734)	$ (6,644)	$ (3,220)