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Divestitures and Acquisitions (Details) - USD ($)
$ / shares in Units, $ in Thousands
3 Months Ended 6 Months Ended 12 Months Ended
Apr. 03, 2018
Mar. 31, 2018
Mar. 31, 2017
Mar. 31, 2018
Mar. 31, 2017
Sep. 30, 2017
Jan. 01, 2017
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]              
Income tax expense       $ 0 $ 6,841    
Income from discontinued operations, net of tax       0 10,994    
Gain on sale of discontinued operations, net of tax   $ 0 $ 2,716 0 2,716    
Net income from discontinued operations   0 2,716 0 13,710    
Summary of Cash Flow Hedge Activity [Abstract]              
Gain (Loss) on Discontinuation of Cash Flow Hedge       0 10,579    
Discontinued Operation, Tax Effect of Gain (Loss) from Disposal of Discontinued Operation   0   $ 0 10,215    
Subsequent Event [Member]              
Subsequent Event [Line Items]              
Subsequent Event, Description
On April 3, 2018, we filed formula rate mechanisms in our Mid-Tex and West Texas service areas, seeking increases in operating income
           
Natural Gas Marketing Segment [Member] | Discontinued Operations, Held-for-sale or Disposed of by Sale [Member]              
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]              
Operating revenues         303,474    
Purchased gas cost         277,554    
Operating expenses         7,874    
Operating income         18,046    
Other nonoperating expense         (211)    
Income from discontinued operations before income taxes         17,835    
Income tax expense         6,841    
Income from discontinued operations, net of tax         10,994    
Gain on sale of discontinued operations, net of tax     2,716   2,716    
Net income from discontinued operations         13,710    
Discontinued Operation, Alternative Cash Flow Information [Abstract]              
Depreciation and Amortization, Discontinued Operations         185    
Capital Expenditure, Discontinued Operations         0    
Noncash Change in Commodity Contract Cash Flow Hedges         (8,165)    
Discontinued Operation, Additional Disclosures [Abstract]              
Gain on Discontinuation of Cash Flow Hedge         10,600    
Gain (Loss) On Hedge Ineffectiveness         3,400    
Disposal Group, Including Discontinued Operation, Consideration Placed in Escrow Released   $ 3,000          
Effect of Fair Value Hedges on Results of Operations [Abstract]              
Commodity contracts         (9,567)    
Fair value adjustment for natural gas inventory designated as the hedged item         12,858    
Total (increase) decrease to purchased gas cost         3,291    
Basis ineffectiveness         (597)    
Timing ineffectiveness         3,888    
Summary of Cash Flow Hedge Activity [Abstract]              
Gain (loss) reclassified from AOCI into purchased gas cost for effective portion of commodity contracts         (2,612)    
Gain (loss) arising from ineffective portion of commodity contracts         111    
Gain (Loss) on Discontinuation of Cash Flow Hedge         10,579    
Total impact on purchased gas cost         8,078    
Gain (Loss) On Derivative Instruments Not Designated Hedges Net Pretax         $ 6,800    
Discontinued Operation, Tax Effect of Gain (Loss) from Disposal of Discontinued Operation     $ 10,215