Fair Value (Reconciliation For All Assets And Liabilities Measured At Fair Value On A Recurring Basis Using Significant Unobservable Inputs (Level 3)) (Details) - Fair Value, Inputs, Level 3 [Member] - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	$ (16,428)	$ (19,410)	$ (27,124)	$ (23,758)
Included in regulatory assets/liabilities	(4,325)	707	(11,906)
Settlements	7,307	7,007	8,540
Natural Gas Exchange Agreements [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	(3,164)	(5,885)	(5,039)	(35)
Included in regulatory assets/liabilities	3,292	259	(6,008)
Settlements	(571)	(1,105)	1,004
Power Exchange Agreements [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	(13,245)	(13,449)	(21,961)	(23,299)
Included in regulatory assets/liabilities	(7,674)	400	(6,198)
Settlements	7,878	8,112	7,536
Commodity Option [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	(19)	(76)	(124)	$ (424)
Included in regulatory assets/liabilities	57	48	300
Settlements	$ 0	$ 0	$ 0

Fair Value (Reconciliation For All Assets And Liabilities Measured At Fair Value On A Recurring Basis Using Significant Unobservable Inputs (Level 3)) (Details) - Fair Value, Inputs, Level 3 [Member] - USD ($)
$ in Thousands

12 Months Ended

Dec. 31, 2017

Dec. 31, 2016

Dec. 31, 2015

Dec. 31, 2014

Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]

Beginning balance

$ (16,428)

$ (19,410)

$ (27,124)

$ (23,758)

Included in regulatory assets/liabilities