Accounting for Derivative Instruments and Hedging Activities (Schedule of Contractual Contingent Liability Positions) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Electric Portfolio
Derivative [Line Items]
Fair Value Liability	$ 19,069	$ 40,914
Collateral Already Posted, Aggregate Fair Value	0	2,639
Contingent Collateral	574	3,187
Credit Rating \| Natural Gas Portfolio
Derivative [Line Items]
Collateral Already Posted, Aggregate Fair Value	1,000
Forward Value of Contract [Member] \| Electric Portfolio
Derivative [Line Items]
Fair Value Liability	0	353
Collateral Already Posted, Aggregate Fair Value	0	2,639
Contingent Collateral	0	0
Requested Credit for Adequate Assurance \| Electric Portfolio
Derivative [Line Items]
Fair Value Liability	18,495	37,374
Collateral Already Posted, Aggregate Fair Value	0	0
Contingent Collateral	0	0
Credit Rating
Derivative [Line Items]
Collateral Already Posted, Aggregate Fair Value	6,400
Credit Rating \| Electric Portfolio
Derivative [Line Items]
Fair Value Liability	574	3,187
Collateral Already Posted, Aggregate Fair Value	0	0
Contingent Collateral	$ 574	$ 3,187

The aggregate fair value of additional assets that would be required to be posted as collateral for derivative instruments with credit-risk-related contingent features if the credit-risk-related contingent features were triggered at the end of the reporting period.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4H
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624258-113959

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- Definition

The aggregate fair value of assets that are already posted, at the end of the reporting period, as collateral for derivative instruments with credit-risk-related contingent features.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4H
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624258-113959

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

The aggregate fair value amounts of derivative instruments that contain credit-risk-related contingent features that are in a net liability position at the end of the reporting period. For nonderivative instruments that are designated and qualify as hedging instruments, the fair value amounts are the carrying value of the nonderivative hedging instrument, including the adjustment for the foreign currency transaction gain (loss) on that instrument.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4H
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624258-113959

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