Costs Related to Purchase Obligations (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Schedule of Costs Related to Purchase Obligations [Line Items]
Total costs related to purchase obligations	$ 212,362	$ 126,135	$ 111,049
Major equipment purchases
Schedule of Costs Related to Purchase Obligations [Line Items]
Total costs related to purchase obligations	43,698	56,723	89,261
Firm transportation expense
Schedule of Costs Related to Purchase Obligations [Line Items]
Total costs related to purchase obligations	59,606	40,274	21,668
Gas drilling obligations
Schedule of Costs Related to Purchase Obligations [Line Items]
Total costs related to purchase obligations	108,167	28,641	0
Other Obligations
Schedule of Costs Related to Purchase Obligations [Line Items]
Total costs related to purchase obligations	$ 891	$ 497	$ 120