| **Acquisitions (Pro Forma Summary) (Details) - Peoples Gas Acquisition [Member] - USD ($) $ in Thousands** | 3 Months Ended |
|------------------------------------------------------------------------------------------------------------|----------------|
||
| **Business Acquisition [Line Items]**                                                                      |           |   |
| Operating revenues                                                                                         | $ 536,653      | $ 598,663 |
| Net income                                                                                                 | $ 137,292      | $ 68,075 |

| X                                                                                                                                                                                                                                                                                                                                                                                                                    |
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| - Definition
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                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
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| - Definition
The pro forma net Income or Loss for the period as if the business combination or combinations had been completed at the beginning of a period.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The pro forma revenue for a period as if the business combination or combinations had been completed at the beginning of the period.
+ References
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| X                                                                                                                                                        |
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