| **Income Taxes (Schedule Of Deferred Tax Assets And Liabilities) (Details) - USD ($) $ in Thousands**                                   | Dec. 31, 2018 | Dec. 31, 2017 |
|-----------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Income Taxes [Abstract]**                                                                                                             |          |          |
| Customers’ advances for construction                                                                                              | $ 13,188      | $ 17,123      |
| Costs expensed for book not deducted for tax, principally accrued expenses                                                              | 27,711        | 12,956        |
| Utility plant acquisition adjustment basis differences                                                                                  | 1,053         | 1,752         |
| Post-retirement benefits                                                                                                                | 39,515        | 36,353        |
| Tax loss and credit carryforwards                                                                                                       | 43,637        | 56,642        |
| Other                                                                                                                                   | 2,761         | 2,348         |
| Total gross deferred tax assets                                                                                                         | 127,865       | 127,174       |
| Less valuation allowance                                                                                                                | 18,082        | 11,623        |
| Net deferred tax assets                                                                                                                 | 109,783       | 115,551       |
| Utility plant, principally due to depreciation and differences in the basis of fixed assets due to variation in tax and book accounting | 837,057       | 795,537       |
| Deferred taxes associated with the gross-up of revenues necessary to recover, in rates, the effect of temporary differences             | 72,258        | 46,143        |
| Tax effect of regulatory asset for post-retirement benefits                                                                             | 39,515        | 36,353        |
| Deferred investment tax credit                                                                                                          | 6,356         | 6,591         |
| Total gross deferred tax liabilities                                                                                                    | 955,186       | 884,624       |
| Net deferred tax liability                                                                                                              | $ 845,403     | $ 769,073     |

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| - Definition
Amount of deferred tax liability attributable to taxable temporary differences.
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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences and carryforwards.
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Amount, before allocation of valuation allowance, of deferred tax asset attributable to deductible temporary differences, classified as other.
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Amount, before allocation of valuation allowance, of deferred tax asset attributable to deductible loss carryforwards, classified as other.
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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from postretirement benefits.
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Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized.
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Amount, after deferred tax asset, of deferred tax liability attributable to taxable differences without jurisdictional netting.
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