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Litigation, Environmental and Other Contingencies Federal Energy Regulatory Commission Proceedings (Details) - Federal Energy Regulatory Commission [Member] - Various Shippers [Member] - Unfavorable Regulatory Action [Member]
$ in Millions
12 Months Ended
Dec. 31, 2018
USD ($)
EPNG [Member] | Opinion 517 issued and implemented (rehearing pending); and Opinion 528 issued. [Member]  
EPNG [Abstract]  
Loss Contingency, Pending Claims, Number 2
Annual Rate Reductions [Member] | SFPP [Member] | Pending Litigation [Member]  
SFPP [Abstract]  
Loss Contingency, Damages Sought, Value $ 30
Revenue Subject to Refund [Member] | SFPP [Member] | Pending Litigation [Member]  
SFPP [Abstract]  
Loss Contingency, Damages Sought, Value $ 330