| **Segment Information (Company's Segment Information, Continuing Operations) (Details) - USD ($) $ in Thousands** | 3 Months Ended | 12 Months Ended |
|-------------------------------------------------------------------------------------------------------------------|----------------|-----------------|
||
| **Segment Reporting Information [Line Items]**                                                                    |           |            |   |   |   |   |   |   |   |   |   |   |   |
| Operating revenues                                                                                                | $ 196,799      | $ 226,593       | $ 203,876 | $ 192,607 | $ 197,067 | $ 221,051 | $ 205,760 | $ 190,326 | $ 819,875 | $ 814,204 | $ 779,903 | $ 761,893 | $ 750,685 |
| Operations and maintenance expense                                                                                | 77,550         | 79,812          | 73,994 | 73,541 | 77,856 | 78,519 | 79,746 | 73,189 | 304,897 | 309,310 | 288,556 |   |   |
| Depreciation                                                                                                      |           |            |   |   |   |   |   |   | 130,987 | 125,290 | 123,054 |   |   |
| Operating income                                                                                                  | 71,962         | $ 97,799        | $ 83,493 | $ 72,331 | 74,615 | $ 95,072 | $ 80,246 | $ 71,167 | 325,585 | 321,100 | 314,359 |   |   |
| Interest expense, net                                                                                             |           |            |   |   |   |   |   |   | 80,594 | 76,536 | 76,397 |   |   |
| Allowance for funds used during construction                                                                      |           |            |   |   |   |   |   |   | 8,815 | 6,219 | 5,134 |   |   |
| Equity (earnings) loss in joint venture                                                                           |           |            |   |   |   |   |   |   | (976) | 35,177 | 3,989 |   |   |
| Income tax (benefit)                                                                                              |           |            |   |   |   |   |   |   | 20,978 | 14,962 | 25,219 | 21,233 | 65,220 |
| Income (loss) from continuing operations                                                                          |           |            |   |   |   |   |   |   | 234,182 | 201,790 | 213,884 | 202,871 | 181,837 |
| Capital expenditures                                                                                              |           |            |   |   |   |   |   |   | 382,996 | 364,689 | 328,605 | 307,908 | 347,098 |
| Total Assets                                                                                                      | 6,158,991      |            |   |   | 5,717,873 |   |   |   | 6,158,991 | 5,717,873 | 5,383,243 | $ 5,027,430 | $ 4,834,165 |
| Goodwill                                                                                                          | 42,208         |            |   |   | 33,866 |   |   |   | 42,208 | 33,866 | 31,184 |   |   |
| Regulated [Member]                                                                                                |           |            |   |   |   |   |   |   |   |   |   |   |   |
| **Segment Reporting Information [Line Items]**                                                                    |           |            |   |   |   |   |   |   |   |   |   |   |   |
| Operating revenues                                                                                                |           |            |   |   |   |   |   |   | 800,107 | 779,613 | 756,057 |   |   |
| Operations and maintenance expense                                                                                |           |            |   |   |   |   |   |   | 285,347 | 282,866 | 274,754 |   |   |
| Depreciation                                                                                                      |           |            |   |   |   |   |   |   | 131,835 | 125,146 | 122,728 |   |   |
| Operating income                                                                                                  |           |            |   |   |   |   |   |   | 326,933 | 315,876 | 305,333 |   |   |
| Interest expense, net                                                                                             |           |            |   |   |   |   |   |   | 76,222 | 72,703 | 72,106 |   |   |
| Allowance for funds used during construction                                                                      |           |            |   |   |   |   |   |   | 8,815 | 6,219 | 5,134 |   |   |
| Income tax (benefit)                                                                                              |           |            |   |   |   |   |   |   | 24,956 | 26,379 | 24,792 |   |   |
| Income (loss) from continuing operations                                                                          |           |            |   |   |   |   |   |   | 234,922 | 224,122 | 213,890 |   |   |
| Capital expenditures                                                                                              |           |            |   |   |   |   |   |   | 381,965 | 363,594 | 325,943 |   |   |
| Total Assets                                                                                                      | 5,953,702      |            |   |   | 5,541,335 |   |   |   | 5,953,702 | 5,541,335 | 5,172,371 |   |   |
| Goodwill                                                                                                          | 37,367         |            |   |   | 27,246 |   |   |   | 37,367 | 27,246 | 24,564 |   |   |
| Other [Member]                                                                                                    |           |            |   |   |   |   |   |   |   |   |   |   |   |
| **Segment Reporting Information [Line Items]**                                                                    |           |            |   |   |   |   |   |   |   |   |   |   |   |
| Operating revenues                                                                                                |           |            |   |   |   |   |   |   | 19,768 | 34,591 | 23,846 |   |   |
| Operations and maintenance expense                                                                                |           |            |   |   |   |   |   |   | 19,550 | 26,444 | 13,802 |   |   |
| Depreciation                                                                                                      |           |            |   |   |   |   |   |   | (848) | 144 | 326 |   |   |
| Operating income                                                                                                  |           |            |   |   |   |   |   |   | (1,348) | 5,224 | 9,026 |   |   |
| Interest expense, net                                                                                             |           |            |   |   |   |   |   |   | 4,372 | 3,833 | 4,291 |   |   |
| Equity (earnings) loss in joint venture                                                                           |           |            |   |   |   |   |   |   | (976) | 35,177 | 3,989 |   |   |
| Income tax (benefit)                                                                                              |           |            |   |   |   |   |   |   | (3,978) | (11,417) | 427 |   |   |
| Income (loss) from continuing operations                                                                          |           |            |   |   |   |   |   |   | (740) | (22,332) | (6) |   |   |
| Capital expenditures                                                                                              |           |            |   |   |   |   |   |   | 1,031 | 1,095 | 2,662 |   |   |
| Total Assets                                                                                                      | 205,289        |            |   |   | 176,538 |   |   |   | 205,289 | 176,538 | 210,872 |   |   |
| Goodwill                                                                                                          | $ 4,841        |            |   |   | $ 6,620 |   |   |   | $ 4,841 | $ 6,620 | $ 6,620 |   |   |

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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.
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| - Definition
The amount of expense recognized in the current period that reflects the allocation of the cost of tangible assets over the assets' useful lives. Includes production and non-production related depreciation.
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| - Definition
Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.
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|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount after tax of income (loss) from continuing operations including portion attributable to the noncontrolling interest.
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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
Amount after tax of income (loss) attributable to nonredeemable noncontrolling shareholder, unit holder, partner, or other equity holder of a joint venture.
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The net result for the period of deducting operating expenses from operating revenues.
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| - Definition
The cash outflow associated with the acquisition of long-lived, physical assets that are used in the normal conduct of business to produce goods and services and not intended for resale; includes cash outflows to pay for construction of self-constructed assets.
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| - Definition
Total increase in earnings in the period representing the cost of equity (based on assumed rate of return) and/or borrowed funds (based on interest rate) used to finance construction of regulated assets, which is expected to be recovered through rate adjustments.
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| - Definition
Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).
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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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Amount of operating expense for routine plant maintenance, repairs and operations of regulated operation.
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Interest expense, net of AFUDC which represents capitalized costs used to finance the construction of utility plants.
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