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Tax Cuts and Jobs Act of 2017 Impacts (Details) - USD ($)
$ in Thousands
 1 Months Ended 3 Months Ended 12 Months Ended
Jan. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
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   $ 908,100    
Regulatory Liability, Excess Deferred Tax   746,200    
One-Time Income Tax Benefit, TCJA   161,884 $ 0  
Deferred Tax Assets, Charitable Contribution Carryforwards [Abstract]        
Deferred Tax Assets, Charitable Contribution Carryforwards   $ 5,100    
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
Regulators in our other services areas, including Texas, Mississippi and Louisiana, have also taken action in response to the TCJA:
• On January 23, 2018, the Railroad Commission of Texas directed the Commission Staff to develop recommendations to ensure that, beginning January 1, 2018, all gas utility customers in Texas receive the full benefit of the TCJA.
•On January 26, 2018, the Mississippi Public Service Commission (MPSC) entered an order requiring each utility to file within thirty days a detailed description identifying how the TCJA will be reflected in the formula rate plan or other rate structures under which the utility operates.
•On January 31, 2018, Louisiana Public Service Commission (LPSC) directed utilities to file reports on February 14, 2018, regarding savings for ratepayers as a result of the new federal tax laws. The LPSC is also considering an accounting order to direct the utilities to track and record the impacts of the TCJA and a rule making docket to address the TCJA.
In January 2018, $3.0 million of this escrowed amount was released and received by the Company.
      
Domestic Tax Authority [Member]        
Operating Loss Carryforwards [Line Items]        
Operating Loss Carryforwards   330,400    
Tax Credit Carryforward, Amount   $ 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   $ 25,900    
Operating Loss Carryforwards, Federal Tax Effects   6,900    
Tax Credit Carryforward, Amount   1,500    
Tax Credit Carryforward, Federal Tax Effects   $ 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