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Tax Cuts and Jobs Act of 2017 Impacts (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended 12 Months Ended
Apr. 03, 2018
Mar. 31, 2018
Mar. 31, 2018
Mar. 31, 2017
Sep. 30, 2017
Income Tax Disclosure [Abstract]          
Federal Statutory Income Tax Rate, Pre TCJA, Percent     35.00%   35.00%
Federal Statutory Income Tax Rate, Post TCJA, Percent     21.00%    
Blended Federal Statutory Income Tax Rate, Percent     24.50%    
Deferred Tax Liabilities, TCJA Deduction     $ 903,500    
Regulatory Liability, Excess Deferred Tax   $ 737,798 737,798   $ 0
One-Time Income Tax Benefit, TCJA   3,800 165,675 $ 0  
Deferred Tax Assets, Charitable Contribution Carryforwards [Abstract]          
Deferred Tax Assets, Charitable Contribution Carryforwards   5,200 $ 5,200    
Charitable Contribution Carryforwards, Expiration Date, Minimum     Dec. 31, 2018    
Charitable Contribution Carryforwards, Expiration Date, Maximum     Dec. 31, 2023    
Valuation Allowance, Tax Credit Carryforward [Member]          
Valuation and Qualifying Accounts Disclosure [Line Items]          
Valuation Allowances and Reserves, Deductions     $ 4,200    
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
        
Domestic Tax Authority [Member]          
Operating Loss Carryforwards [Line Items]          
Operating Loss Carryforwards   274,700 274,700    
Tax Credit Carryforward, Amount   10,100 $ 10,100    
Domestic Tax Authority [Member] | Minimum [Member]          
Operating Loss Carryforwards [Line Items]          
Operating Loss Carryforwards, Expiration Date     Dec. 31, 2029    
Tax Credit Carryforward, Expected Refund Date     Dec. 31, 2019    
Domestic Tax Authority [Member] | Maximum [Member]          
Operating Loss Carryforwards [Line Items]          
Tax Credit Carryforward, Expected Refund Date     Dec. 31, 2022    
State and Local Jurisdiction [Member]          
Operating Loss Carryforwards [Line Items]          
Operating Loss Carryforwards   21,500 $ 21,500    
Operating Loss Carryforwards, Federal Tax Effects   5,700 5,700    
Tax Credit Carryforward, Amount   1,500 1,500    
Tax Credit Carryforward, Federal Tax Effects   $ 400 $ 400    
State and Local Jurisdiction [Member] | Minimum [Member]          
Operating Loss Carryforwards [Line Items]          
Operating Loss Carryforwards, Expiration Date     Dec. 31, 2018    
State and Local Jurisdiction [Member] | Maximum [Member]          
Operating Loss Carryforwards [Line Items]          
Operating Loss Carryforwards, Expiration Date     Dec. 31, 2032