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Risk Management Activities and Derivative Transactions Counterparty Credit Risk (Details)
$ in Millions
Dec. 31, 2018
USD ($)
megawatts
Derivative  
Total derivatives not subject to master netting or similar arrangement | $ $ 77 [1]
Long-term Contract for Purchase of Electric Power [Domain]  
Derivative  
Megawatts | megawatts 440
[1] There are no derivative contracts subject to a master netting arrangement or similar agreement that are not offset in the consolidated balance sheets.