Disclosures About Derivative Instruments and Hedging Activities (Details)	Sep. 30, 2013 DTH	Sep. 30, 2012 DTH
Natural Gas (millions of kilowatt hours) [Member]
Outstanding derivative commodity instruments volumes
Outstanding derivative commodity instruments volumes	24,300,000	23,600,000
Electricity (millions of kilowatt-hours) [Member] | Call Option [Member]
Outstanding derivative commodity instruments volumes
Outstanding derivative commodity instruments volumes	754,400,000	1,415,700,000
Electricity (millions of kilowatt-hours) [Member] | Put Option [Member]
Outstanding derivative commodity instruments volumes
Outstanding derivative commodity instruments volumes	393,000,000	135,300,000
Designated as Hedging Instrument [Member] | LPG (millions of gallons) [Member]
Outstanding derivative commodity instruments volumes
Outstanding derivative commodity instruments volumes	279,000,000	243,900,000
Designated as Hedging Instrument [Member] | Electricity (millions of kilowatt-hours) [Member] | Call Option [Member]
Outstanding derivative commodity instruments volumes
Outstanding derivative commodity instruments volumes	594,800,000	1,151,700,000