Financial Instruments Descriptions (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013 MMcf	Jun. 30, 2012	Sep. 30, 2012
General Discussion Of Derivative Instruments And Hedging Activities [Abstract]
Minimum Hedge Percentage Of Anticipated Heating Season Gas			25.00%
Maximum Hedge Percentage Of Anticipated Heating Season Gas			50.00%
Hedge Percentage Of Actual Heating Season Gas			33.00%
Hedge Volume Of Actual Heating Season Gas			22,800
Minimum Length Of Time Hedged In Cash Flow Hedge			0 years 1 month 0 days
Maximum Length Of Time Hedged In Cash Flow Hedge			4 years 10 months 0 days
Derivative, Collateral, Right to Reclaim Cash	$ 14.3		$ 14.3		$ 23.7
Gain Loss On Hedge Ineffectiveness	(0.4)	19.0	17.3	21.2
Regulated Segments Effective Income Tax Rate Reconciliation At Federal Statutory Income Tax Rate			37.00%
Nonregulated Segment Effective Income Tax Rate Reconciliation At Federal Statutory Income Tax Rate			39.00%
Gain Loss On Derivative Instruments Not Designated Hedges Net Pretax	(8.4)	11.2	(1.7)	(3.8)
Natural Gas Inventory [Abstract]
Inventory write-down to lower of cost or market				1.7
Interest Rate Hedges, Senior Notes 2013 Issuance [Member]
Derivative [Line Items]
Notional Amount of Interest Rate Cash Flow Hedge Derivatives	350		350
Unrealized Gain (Loss) on Interest Rate Cash Flow Hedges, Pretax, Accumulated Other Comprehensive Income (Loss)	66.6		66.6
Interest Rate Hedges, Senior Notes 2015 Issuance [Member]
Derivative [Line Items]
Notional Amount of Interest Rate Cash Flow Hedge Derivatives	500		500
Interest Rate Hedges, Short-term Note 2012 Issuance [Member]
Derivative [Line Items]
Notional Amount of Interest Rate Cash Flow Hedge Derivatives	260		260
Interest Rate Hedges, Senior Notes 2017 Issuance [Member]
Derivative [Line Items]
Notional Amount of Interest Rate Cash Flow Hedge Derivatives	$ 250		$ 250