| **Business Segments (Details) $ in Thousands**                        | 3 Months Ended | 9 Months Ended |
|-----------------------------------------------------------------------|----------------|----------------|
||
| **Segment Reporting Information [Line Items]**                        |                |           |   |   |   |
| Number of reportable segments | segment                               |                |           |   | 3 |   |
| **Segment Reporting Information [Abstract]**                          |                |           |   |   |   |
| Revenues from external customers                                      | [1]            | $ 157,806      | $ 158,266 | $ 969,329 | $ 1,190,462 |
| Margin                                                                |                | 114,771        | 111,572 | 625,385 | 607,263 |
| Operations and maintenance expenses                                   |                | 68,941         | 69,655 | 215,850 | 207,328 |
| Income from equity method investments                                 |                | 4,238          | 5,801 | 28,025 | 29,908 |
| Operating income (loss) before income taxes                           |                | 119            | (2,021) | 275,201 | 271,354 |
| Income (loss) before income taxes for reportable segments             |                | (11,193)       | (13,260) | 253,241 | 247,980 |
| **Segment Reporting Information, Operating Income (Loss) [Abstract]** |                |           |   |   |   |
| Segment operating income (loss) before income taxes                   |                | 119            | (2,021) | 275,201 | 271,354 |
| Utility income taxes                                                  |                | 6,339          | 7,097 | (87,660) | (85,583) |
| Operating income                                                      |                | 6,655          | 5,233 | 187,927 | 186,114 |
| **Segment Reporting Information, Profit (Loss) [Abstract]**           |                |           |   |   |   |
| Income (loss) before income taxes for reportable segments             |                | (11,193)       | (13,260) | 253,241 | 247,980 |
| Income taxes                                                          |                | 4,463          | 5,000 | (98,749) | (96,860) |
| Net Income (Loss)                                                     |                | (6,730)        | (8,260) | 154,492 | 151,120 |
| Regulated Utility                                                     |                |           |   |   |   |
| **Segment Reporting Information [Abstract]**                          |                |           |   |   |   |
| Revenues from external customers                                      |                | 157,806        | 158,266 | 969,329 | 1,190,462 |
| Margin                                                                |                | 114,771        | 111,572 | 625,385 | 607,263 |
| Operations and maintenance expenses                                   |                | 68,867         | 69,587 | 215,675 | 207,162 |
| Income from equity method investments                                 |                | 0              | 0 | 0 | 0 |
| Operating income (loss) before income taxes                           |                | 316            | (1,864) | 275,587 | 271,697 |
| Income (loss) before income taxes for reportable segments             |                | (15,233)       | (18,904) | 225,601 | 218,414 |
| **Segment Reporting Information, Operating Income (Loss) [Abstract]** |                |           |   |   |   |
| Segment operating income (loss) before income taxes                   |                | 316            | (1,864) | 275,587 | 271,697 |
| **Segment Reporting Information, Profit (Loss) [Abstract]**           |                |           |   |   |   |
| Income (loss) before income taxes for reportable segments             |                | (15,233)       | (18,904) | 225,601 | 218,414 |
| Regulated Non-Utility Activities                                      |                |           |   |   |   |
| **Segment Reporting Information [Abstract]**                          |                |           |   |   |   |
| Revenues from external customers                                      |                | 0              | 0 | 0 | 0 |
| Margin                                                                |                | 0              | 0 | 0 | 0 |
| Operations and maintenance expenses                                   |                | 16             | 19 | 49 | 70 |
| Income from equity method investments                                 |                | 1,229          | 3,894 | 9,495 | 11,242 |
| Operating income (loss) before income taxes                           |                | (121)          | (89) | (154) | (140) |
| Income (loss) before income taxes for reportable segments             |                | 1,108          | 3,805 | 9,341 | 11,102 |
| **Segment Reporting Information, Operating Income (Loss) [Abstract]** |                |           |   |   |   |
| Segment operating income (loss) before income taxes                   |                | (121)          | (89) | (154) | (140) |
| **Segment Reporting Information, Profit (Loss) [Abstract]**           |                |           |   |   |   |
| Income (loss) before income taxes for reportable segments             |                | 1,108          | 3,805 | 9,341 | 11,102 |
| Unregulated Non-Utility Activities                                    |                |           |   |   |   |
| **Segment Reporting Information [Abstract]**                          |                |           |   |   |   |
| Revenues from external customers                                      |                | 0              | 0 | 0 | 0 |
| Margin                                                                |                | 0              | 0 | 0 | 0 |
| Operations and maintenance expenses                                   |                | 58             | 49 | 126 | 96 |
| Income from equity method investments                                 |                | 3,009          | 1,907 | 18,530 | 18,666 |
| Operating income (loss) before income taxes                           |                | (76)           | (68) | (232) | (203) |
| Income (loss) before income taxes for reportable segments             |                | 2,932          | 1,839 | 18,299 | 18,464 |
| **Segment Reporting Information, Operating Income (Loss) [Abstract]** |                |           |   |   |   |
| Segment operating income (loss) before income taxes                   |                | (76)           | (68) | (232) | (203) |
| **Segment Reporting Information, Profit (Loss) [Abstract]**           |                |           |   |   |   |
| Income (loss) before income taxes for reportable segments             |                | $ 2,932        | $ 1,839 | $ 18,299 | $ 18,464 |
|                                                                       |
| [1]See Note 13 for amounts attributable to affiliates.                |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The component of income tax expense for the period representing amounts of income taxes paid or payable as determined by applying the provisions for relevant enacted tax laws to relevant amounts of utility taxable income (loss).
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Amount of operating income or loss for the reportable segment before income taxes and extraordinary items
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Segment reporting information on operating income (loss) and other items.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Aggregate revenue less cost of goods and services sold or operating expenses directly attributable to the revenue generation activity.
+ References
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of income (loss) from continuing operations, including income (loss) from equity method investments, before deduction of income tax expense (benefit), and income (loss) attributable to noncontrolling interest.
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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the entity's proportionate share for the period of the net income (loss) of its investee (such as unconsolidated subsidiaries and joint ventures) to which the equity method of accounting is applied. This item includes income or expense related to stock-based compensation based on the investor's grant of stock to employees of an equity method investee.
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| - Definition
Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.
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|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.
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| - Definition
Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.
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| - Definition
Generally recurring costs associated with normal operations except for the portion of these expenses which can be clearly related to production and included in cost of sales or services. Includes selling, general and administrative expense.
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| - Definition
The net result for the period of deducting operating expenses from operating revenues.
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The total amount of operating revenues recognized during the period.
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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.
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