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Financial Instruments Descriptions (Details)
$ in Millions
3 Months Ended 6 Months Ended
Mar. 31, 2016
USD ($)
Mar. 31, 2017
USD ($)
MMcf
Mar. 31, 2016
USD ($)
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   27.00%  
Hedge Volume Of Actual Heating Season Gas | MMcf   16,200  
Gain on Discontinuation of Cash Flow Hedge   $ 10.6  
Accumulated Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Interest Rate Agreements, Realized Gain (Loss), Net of Tax   (18.1)  
Gain (Loss) On Hedge Ineffectiveness $ (3.3) $ 3.4 $ 4.6
Regulated Segments Effective Income Tax Rate Reconciliation At Federal Statutory Income Tax Rate   37.00% 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 $ 2.5 $ 6.8 $ (4.7)
Interest Rate Hedges, Senior Notes 2017 Issuance [Member]      
Derivative [Line Items]      
Notional Amount of Interest Rate Cash Flow Hedge Derivatives   250.0  
Anticipated Debt Issuance, Amount   $ 250.0  
Derivative, Average Forward Interest Rate   3.37%  
Interest Rate Hedges, Senior Notes 2019 Issuance [Member]      
Derivative [Line Items]      
Notional Amount of Interest Rate Cash Flow Hedge Derivatives   $ 450.0  
Anticipated Debt Issuance, Amount   $ 450.0  
Derivative, Average Forward Interest Rate   3.78%