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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 |  |

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