Income Taxes (Schedule Of Deferred Tax Assets And Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Income Taxes [Abstract]
Customers’ advances for construction	$ 22,664	$ 13,188
Costs expensed for book not deducted for tax, principally accrued expenses	1,473	27,711
Utility plant acquisition adjustment basis differences		1,053
Post-retirement benefits	20,575	39,515
Tax loss and credit carryforwards	65,438	43,637
Operating lease liabilities	3,540
Other	2,798	2,761
Total gross deferred tax assets	116,488	127,865
Less valuation allowance	22,873	18,082
Net deferred tax assets	93,615	109,783
Utility plant, principally due to depreciation and differences in the basis of fixed assets due to variation in tax and book accounting	909,219	837,057
Deferred taxes associated with the gross-up of revenues necessary to recover, in rates, the effect of temporary differences	101,126	72,258
Tax effect of regulatory asset for post-retirement benefits	8,973	39,515
Utility plant acquisition adjustment basis differences	827
Deferred investment tax credit	6,088	6,356
Operating lease right-of-use assets	3,540
Total gross deferred tax liabilities	1,029,773	955,186
Net deferred tax liability	$ 936,158	$ 845,403