Costs Related to Purchase Obligations (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Schedule of Costs Related to Purchase Obligations [Line Items]
Total costs related to purchase obligations	$ 64,940	$ 105,808	$ 228,186	$ 240,375
Major equipment purchases
Schedule of Costs Related to Purchase Obligations [Line Items]
Total costs related to purchase obligations	8,990	59,799	57,571	104,980
Firm transportation expense
Schedule of Costs Related to Purchase Obligations [Line Items]
Total costs related to purchase obligations	29,654	18,844	89,196	49,711
Gas drilling obligations
Schedule of Costs Related to Purchase Obligations [Line Items]
Total costs related to purchase obligations	26,296	27,100	81,419	85,192
Other Obligations
Schedule of Costs Related to Purchase Obligations [Line Items]
Total costs related to purchase obligations	$ 0	$ 65	$ 0	$ 492