Disclosures About Derivative Instruments and Hedging Activities (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013	Mar. 31, 2012
Designated as Hedging Instrument [Member] | LPG (millions of gallons) [Member]
Outstanding derivative commodity instruments volumes
Outstanding derivative commodity instruments volumes	129,400,000	146,800,000	129,400,000	146,800,000
Designated as Hedging Instrument [Member] | Natural gas (millions of dekatherms) [Member]
Outstanding derivative commodity instruments volumes
Outstanding derivative commodity instruments volumes	17,300,000	22,800,000	17,300,000	22,800,000
Designated as Hedging Instrument [Member] | Electricity (millions of kilowatt-hours) [Member] | Forward Purchase Contracts [Member]
Outstanding derivative commodity instruments volumes
Outstanding derivative commodity instruments volumes	1,034,400,000	1,793,900,000	1,034,400,000	1,793,900,000
Designated as Hedging Instrument [Member] | Electricity (millions of kilowatt-hours) [Member] | Forward Sales Contracts [Member]
Outstanding derivative commodity instruments volumes
Outstanding derivative commodity instruments volumes	449,900,000	153,600,000	449,900,000	153,600,000
Cash Flow Hedges [Member]
Derivative [Line Items]
Derivative Instruments Gain Loss Recognized in Other Comprehensive Income and Noncontrolling Interests Effective Portion	$ 16.1	$ (54.7)	$ 2.6	$ (119.6)
Interest Rate Contracts [Member] | Cash Flow Hedges [Member] | Interest Expense/Other Income [Member]
Derivative [Line Items]
Derivative Instruments Gain Loss Recognized in Other Comprehensive Income and Noncontrolling Interests Effective Portion	$ 8.0	$ (2.8)	$ 9.0	$ (12.5)