| **Income Taxes (Details) - USD ($) $ in Thousands**              | 3 Months Ended | 12 Months Ended |
|------------------------------------------------------------------|----------------|-----------------|
||
| **Income Taxes**                                                 |           |            |
| Reduction in net deferred income tax liabilities                 | $ 11,000       | $ 1,140,000     |
| Regulatory liabilities, non-current                              | 2,410,597      | 2,452,536       |
| APS                                                              |           |            |
| **Income Taxes**                                                 |           |            |
| Reduction in net deferred income tax liabilities                 |           | 1,140,000       |
| Regulatory liabilities, non-current                              | 2,410,597      | 2,452,536       |
| Gross-up for revenue requirement of rate regulation              |           | 377,000         |
| Excess deferred income taxes - ACC - Tax Cuts and Jobs Act | APS |           |            |
| **Income Taxes**                                                 |           |            |
| Regulatory liabilities, non-current                              | $ 9,000        | $ 1,520,000     |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Public Utilities, Requested Rate Increase (Decrease), Amount, Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Provisional Income Tax Expense (Benefit)
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Tax Cuts And Jobs Act Of 2017, Incomplete Accounting, Change In Tax Rate, Provisional Income Tax Expense (Benefit)
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No definition available.
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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount for the individual regulatory noncurrent liability as itemized in a table of regulatory noncurrent liabilities as of the end of the period.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.24) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682
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| X                                                                                                                                                 |
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