Fair Value Measurements--Concentrations of Credit Risk (Details)	12 Months Ended
	Dec. 31, 2014 customers	Dec. 31, 2013 customers	Dec. 31, 2012 customers
Counterparty Credit Risk [Member]
Concentration Risk [Line Items]
Number of counterparties to derivative contracts	10
Number of counterparties to derivative contracts subject to offset under credit facility	10
Customer Concentration Risk [Member]
Concentration Risk [Line Items]
Number of significant purchasers	2	2	3
Commodity Sales [Member] | Customer A [Member]
Concentration Risk [Line Items]
Percentage of sales to significant purchaser	23.70%	20.10%	19.60%
Commodity Sales [Member] | Customer B [Member]
Concentration Risk [Line Items]
Percentage of sales to significant purchaser	14.00%	19.10%	13.90%
Commodity Sales [Member] | Customer C [Member]
Concentration Risk [Line Items]
Percentage of sales to significant purchaser			12.70%