Derivatives (Details 1) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012
Derivatives, Fair Value [Line Items]
Commodity derivative assets	$ 38,352	$ 61,247	$ (73,424)
Commodity derivative liabilities	0	0	0
Commodity derivative assets - current	105,390	103,105	166,264
Commodity derivative assets - noncurrent	5	1,906	7,047
Commodity derivative liabilities - current	(88,234)	(33,277)	(43,589)
Commodity derivative liabilities - noncurrent	(9,636)	(1,134)	(590)
Commodity
Derivatives, Fair Value [Line Items]
Commodity derivative assets	92,519	144,541	205,750
Commodity derivative liabilities	(112,095)	(60,169)	(164,864)
Cash collateral	27,101	(13,772)	88,246
Total	7,525	70,600	129,132
Commodity | Commodity derivative assets - current
Derivatives, Fair Value [Line Items]
Commodity derivative assets	89,540	137,119	194,771
Commodity derivative liabilities	(11,251)	(20,242)	(116,753)
Cash collateral	27,101	(13,772)	88,246
Commodity derivative assets - current	105,390	103,105	166,264
Commodity | Commodity derivative assets - noncurrent
Derivatives, Fair Value [Line Items]
Commodity derivative assets	5	2,059	7,383
Commodity derivative liabilities	0	(153)	(336)
Cash collateral	0	0	0
Commodity derivative assets - noncurrent	5	1,906	7,047
Commodity | Commodity derivative liabilities - current
Derivatives, Fair Value [Line Items]
Commodity derivative assets	2,933	5,233	3,558
Commodity derivative liabilities	(91,167)	(38,510)	(47,147)
Cash collateral	0	0	0
Commodity derivative liabilities - current	(88,234)	(33,277)	(43,589)
Commodity | Commodity derivative liabilities - noncurrent
Derivatives, Fair Value [Line Items]
Commodity derivative assets	41	130	38
Commodity derivative liabilities	(9,677)	(1,264)	(628)
Cash collateral	0	0	0
Commodity derivative liabilities - noncurrent	$ (9,636)	$ (1,134)	$ (590)