Accounting for Derivative Instruments and Hedging Activities (Narrative) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Electric Portfolio
Derivative [Line Items]
Collateral Already Posted, Aggregate Fair Value	$ 0	$ 2,639
Credit Rating | Natural Gas Portfolio
Derivative [Line Items]
Collateral Already Posted, Aggregate Fair Value	$ 1,000
External Credit Rating, Investment Grade [Member] | Electric Portfolio
Derivative [Line Items]
Percentage of derivatives with credit risk exposure	95.80%
External Credit Rating, Non Investment Grade [Member] | Electric Portfolio
Derivative [Line Items]
Percentage of derivatives with credit risk exposure	4.20%
Credit Rating
Derivative [Line Items]
Collateral Already Posted, Aggregate Fair Value	$ 6,400
Credit Rating | Electric Portfolio
Derivative [Line Items]
Collateral Already Posted, Aggregate Fair Value	$ 0	$ 0