Fair Value Measurements--Concentrations of Credit Risk (Details)	12 Months Ended
	Dec. 31, 2013 customers	Dec. 31, 2012 customers	Dec. 31, 2011 customers
Counterparty Credit Risk
Concentration Risk [Line Items]
Number of counterparties to derivative contracts	12
Number of counterparties to derivative contracts subject to offset under credit facility	11
Customer Concentration Risk
Concentration Risk [Line Items]
Number of significant purchasers	2	3	3
Commodity Sales | Customer A [Member]
Concentration Risk [Line Items]
Percentage of sales to significant purchaser	20.10%	19.60%	15.10%
Commodity Sales | Customer B [Member]
Concentration Risk [Line Items]
Percentage of sales to significant purchaser	19.10%	13.90%	13.60%
Commodity Sales | Customer C [Member]
Concentration Risk [Line Items]
Percentage of sales to significant purchaser		12.70%	10.40%