Fair Value Measurement (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commodity derivatives	$ 4,128	$ 133
Long-term investments	962	937
Total	5,090	1,070
Commodity derivatives	44,302	50,033
Interest-rate swap derivatives	79,725	24,346
Total	124,027	74,379
Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commodity derivatives	0	0
Long-term investments	962	937
Total	962	937
Commodity derivatives	0	0
Interest-rate swap derivatives	0	0
Total	0	0
Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commodity derivatives	4,128	133
Long-term investments	0	0
Total	4,128	133
Commodity derivatives	44,302	50,033
Interest-rate swap derivatives	79,725	24,346
Total	124,027	74,379
Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commodity derivatives	0	0
Long-term investments	0	0
Total	0	0
Commodity derivatives	0	0
Interest-rate swap derivatives	0	0
Total	$ 0	$ 0