Derivatives (Details)	Jun. 30, 2013 ft3	Dec. 31, 2012 ft3	Mar. 31, 2012 Natural Gas - Swaps December 2011- March 2012 (in cubic feet)	Nov. 30, 2012 Natural Gas - Swaps January 2013- March 2014 (in cubic feet)
Cash-Flow Hedges
Daily volume			50,000,000	50,000,000
Average Price (in dollars per cubic foot)			0.006070	0.004300
Natural gas held in storage (in cubic feet)	13,000,000,000	20,000,000,000
Forecast sale of natural gas from storage designated as cash-flow hedges (in cubic feet)	15,000,000,000	20,000,000,000