Regulatory Assets And Liabilities (Components Of Regulatory Assets And Regulatory Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Regulatory Of Assets And Liabilities [Line Items]
Regulatory Assets	$ 878,132	$ 788,076
Regulatory Liabilities	517,599	531,027
Income Taxes [Member]
Regulatory Of Assets And Liabilities [Line Items]
Regulatory Assets	736,120	657,378
Regulatory Liabilities	389,424	414,787
Customer Refunds Resulting From TCJA [Member]
Regulatory Of Assets And Liabilities [Line Items]
Regulatory Liabilities	3,907	4,593
Utility Plant Retirement Costs [Member]
Regulatory Of Assets And Liabilities [Line Items]
Regulatory Assets	7,873	6,743
Regulatory Liabilities	43,742	38,435
Postretirement Benefits [Member]
Regulatory Of Assets And Liabilities [Line Items]
Regulatory Assets	110,661	110,719
Regulatory Liabilities	78,557	71,285
Accrued Vacation [Member]
Regulatory Of Assets And Liabilities [Line Items]
Regulatory Assets	2,439	2,447
Water Tank Painting [Member]
Regulatory Of Assets And Liabilities [Line Items]
Regulatory Assets	6,175	2,864
Regulatory Liabilities	1,928	1,855
Fair Value Adjustment Of Long-Term Debt Assumed In Acquisition [Member]
Regulatory Of Assets And Liabilities [Line Items]
Regulatory Assets	2,166	2,533
Debt Refinancing [Member]
Regulatory Of Assets And Liabilities [Line Items]
Regulatory Assets	6,564
Rate Case Filing Expenses And Other [Member]
Regulatory Of Assets And Liabilities [Line Items]
Regulatory Assets	6,134	5,392
Regulatory Liabilities	$ 41	$ 72