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Divestitures and Acquisitions (Details) - USD ($)
$ / shares in Units, $ in Thousands
3 Months Ended 12 Months Ended
Jan. 03, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2015
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2015
Jan. 01, 2017
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]                          
Income from discontinued operations, net of tax, before gain on disposal                   $ 10,994 $ 4,562 $ 9,452  
Gain on sale from discontinued operations, net of tax                   2,716 0 0  
Net income from discontinued operations, net of tax                   $ 13,710 4,562 9,452  
Discontinued Operations, Significant Accounting Policies [Abstract]                          
Derivative, Cash Collateral           $ 50,400         50,400    
Natural Gas Marketing Segment [Member]                          
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]                          
Operating revenues           244,876 $ 176,704 $ 242,253 $ 231,468        
Purchased gas cost           238,448 160,889 236,993 221,999        
Operating income           184 8,768 (1,640) 3,476        
Income from discontinued operations, net of tax, before gain on disposal   $ 0 $ 0 $ 0 $ 10,994 (610) $ 5,050 $ (1,193) $ 1,315        
Gain on sale from discontinued operations, net of tax   $ 0 $ 0 $ 2,716 $ 0                
Natural Gas Marketing Segment [Member] | Discontinued Operations, Held-for-sale or Disposed of by Sale [Member]                          
Disposal Group, Including Discontinued Operation, Additional Disclosures [Abstract]                          
Disposal Group, Including Discontinued Operation, Cash Purchase Price                         $ 38,300
Disposal Group, Including Discontinued Operation, Working Capital Adjustment                         109,000
Disposal Group, Including Discontinued Operation, Consideration                         147,300
Disposal Group, Including Discontinued Operation, Consideration Placed in Escrow                         $ 7,000
Discontinued Operation, Gain (Loss) on Disposal of Discontinued Operation, Net of Tax, Per Diluted Share                   $ 0.03      
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]                          
Operating revenues                   $ 303,474 1,005,090 1,409,071  
Purchased gas cost                   277,554 968,118 1,359,832  
Operating expenses                   7,874 26,184 30,076  
Operating income                   18,046 10,788 19,163  
Other nonoperating expense                   (211) (2,495) (3,570)  
Income from discontinued operations before income taxes                   17,835 8,293 15,593  
Income tax expense                   6,841 3,731 6,141  
Income from discontinued operations, net of tax, before gain on disposal                   10,994 4,562 9,452  
Gain on sale from discontinued operations, net of tax                   2,716 0 0  
Discontinued Operation, Tax Effect of Gain (Loss) from Disposal of Discontinued Operation                   10,215 0 0  
Net income from discontinued operations, net of tax                   13,710 4,562 9,452  
Assets [Abstract]                          
Total assets of the disposal group           216,715         216,715    
Liabilities [Abstract]                          
Total liabilities of the disposal group           150,118         150,118    
Discontinued Operation, Alternative Cash Flow Information [Abstract]                          
Depreciation and Amortization, Discontinued Operations                   185 2,304 2,388  
Capital Expenditure, Discontinued Operations                   0 321 226  
Noncash Change in Commodity Contract Cash Flow Hedges                   $ (8,165) (33,533) $ 38,956  
Discontinued Operations, Significant Accounting Policies [Abstract]                          
Derivative, Cash Collateral           24,700         24,700    
Natural Gas Marketing Segment [Member] | Discontinued Operations, Held-for-sale or Disposed of by Sale [Member] | Uncommitted Revolving Credit Facility [Member] | $25 Million Uncommitted Facility [Member]                          
Discontinued Operations, Significant Accounting Policies [Abstract]                          
Extinguishment of Debt, Amount $ 25,000                        
Natural Gas Marketing Segment [Member] | Discontinued Operations, Held-for-sale or Disposed of by Sale [Member] | Unsecured Revolving Credit Note [Member] | $15 Million Revolving Credit Note [Member]                          
Discontinued Operations, Significant Accounting Policies [Abstract]                          
Extinguishment of Debt, Amount $ 15,000                        
Natural Gas Marketing Segment [Member] | Discontinued Operations, Held-for-sale or Disposed of by Sale [Member] | Minimum [Member]                          
Discontinued Operations, Significant Accounting Policies [Abstract]                          
Property, Plant and Equipment, Useful Life                   3 years      
Natural Gas Marketing Segment [Member] | Discontinued Operations, Held-for-sale or Disposed of by Sale [Member] | Maximum [Member]                          
Discontinued Operations, Significant Accounting Policies [Abstract]                          
Property, Plant and Equipment, Useful Life                   30 years      
Natural Gas Marketing Segment [Member] | Discontinued Operations, Held-for-sale or Disposed of by Sale [Member] | Disposal Group Assets and Liabilities Held for Sale [Member]                          
Assets [Abstract]                          
Net property, plant and equipment           11,905         11,905    
Accounts receivable           93,551         93,551    
Gas stored underground           54,246         54,246    
Assets from risk management activities           8,743         8,743    
Other current assets           5,968         5,968    
Goodwill           16,445         16,445    
Noncurrent assets from risk management activities           169         169    
Deferred charges and other assets           266         266    
Total assets of the disposal group [1]           191,293         191,293    
Liabilities [Abstract]                          
Accounts payable and accrued liabilities           72,268         72,268    
Other current liabilities           9,640         9,640    
Deferred credits and other           316         316    
Total liabilities of the disposal group [1]           82,224         82,224    
Natural Gas Marketing Segment [Member] | Discontinued Operations, Held-for-sale or Disposed of by Sale [Member] | Disposal Group Assets and Liabilities Not Held for Sale [Member]                          
Assets [Abstract]                          
Cash           25,417         25,417    
Other assets           5         5    
Liabilities [Abstract]                          
Intercompany note payable           35,000         35,000    
Tax liabilities           15,471         15,471    
Intercompany payables           14,139         14,139    
Other liabilities           $ 3,284         $ 3,284    
Enlink North Texas Pipeline, LP [Member]                          
Business Acquisition [Line Items]                          
Business Acquisition, Date of Acquisition Agreement                   Dec. 20, 2016      
Payments to Acquire Businesses, Gross                   $ 85,000      
Other Payments to Acquire Businesses                   $ 1,100      
[1] Amounts are classified as current and long term in the statement of financial position.