Financial Instruments Descriptions (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014 MMcf	Mar. 31, 2013
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			32.00%
Hedge Volume Of Actual Heating Season Gas			24,600
Minimum Length Of Time Hedged In Cash Flow Hedge	1 month
Maximum Length Of Time Hedged In Cash Flow Hedge	49 months
Gain (Loss) On Hedge Ineffectiveness	$ (3.7)	$ 1.7	$ 1.4	$ 17.8
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	(9.3)	6.8	(10.1)	6.7
Interest Rate Hedges, Senior Notes 2015 Issuance [Member]
Derivative [Line Items]
Notional Amount of Interest Rate Cash Flow Hedge Derivatives	500		500
Anticipated Debt Issuance, Amount	500		500
Derivative, Average Forward Interest Rate	3.129%		3.129%
Interest Rate Hedges, Senior Notes 2017 Issuance [Member]
Derivative [Line Items]
Notional Amount of Interest Rate Cash Flow Hedge Derivatives	250		250
Anticipated Debt Issuance, Amount	250		250
Derivative, Average Forward Interest Rate	3.37%		3.37%
Interest Rate Hedges, Senior Notes 2019 Issuance [Member]
Derivative [Line Items]
Notional Amount of Interest Rate Cash Flow Hedge Derivatives	250		250
Anticipated Debt Issuance, Amount	$ 450		$ 450
Derivative, Average Forward Interest Rate	3.95%		3.95%