Derivatives (Details)	Sep. 30, 2013 ft3	Dec. 31, 2012 ft3	Nov. 30, 2012 Natural Gas - Swaps through March 2014 (in cubic feet)	Mar. 31, 2012 Natural Gas - Swaps through March 2012 (in cubic feet)
Cash-Flow Hedges
Daily volume			50,000,000	50,000,000
Weighted-average strike price in dollars per cubic foot)			0.00430	0.00607
Natural gas held in storage (in cubic feet)	16,000,000,000	20,000,000,000
Forecast sale of natural gas from storage designated as cash-flow hedges (in cubic feet)	18,000,000,000	20,000,000,000