Costs Related to Purchase Obligations (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Schedule of Costs Related to Purchase Obligations [Line Items]
Unrecorded Unconditional Purchase Obligation, Purchases	$ 249,043	$ 291,309	$ 202,883
Major equipment purchases
Schedule of Costs Related to Purchase Obligations [Line Items]
Unrecorded Unconditional Purchase Obligation, Purchases	12,668	101,367	34,219
Firm transportation expense
Schedule of Costs Related to Purchase Obligations [Line Items]
Unrecorded Unconditional Purchase Obligation, Purchases	126,766	78,475	59,606
Gas drilling obligations
Schedule of Costs Related to Purchase Obligations [Line Items]
Unrecorded Unconditional Purchase Obligation, Purchases	109,609	110,975	108,167
Other Obligations
Schedule of Costs Related to Purchase Obligations [Line Items]
Unrecorded Unconditional Purchase Obligation, Purchases	$ 0	$ 492	$ 891