Disclosures About Derivative Instruments and Hedging Activities (Details)	Mar. 31, 2012 gal	Mar. 31, 2011 gal
LPG [Member]
Outstanding derivative commodity instruments volumes
Outstanding derivative commodity instruments volumes	146,800,000	47,300,000
Natural gas [Member]
Outstanding derivative commodity instruments volumes
Outstanding derivative commodity instruments volumes	22,800,000	21,900,000
Electricity (millions of kilowatt-hours) [Member] | Calls [Member]
Outstanding derivative commodity instruments volumes
Outstanding derivative commodity instruments volumes	1,793,900,000	1,516,200,000
Electricity (millions of kilowatt-hours) [Member] | Puts [Member]
Outstanding derivative commodity instruments volumes
Outstanding derivative commodity instruments volumes	153,600,000