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Federal Energy Regulatory Commission Proceedings (Details) - Federal Energy Regulatory Commission [Member] - Various Shippers [Member] - Unfavorable Regulatory Action [Member]
$ in Millions
6 Months Ended
Jun. 30, 2016
USD ($)
EPNG [Member] | Opinion 517 issued and implemented (rehearing pending); and Opinion 528 issued and is awaiting filing of court document) [Member]  
EPNG [Abstract]  
Loss Contingency, Pending Claims, Number 2
Reparations, Refunds, and Rate Reductions [Member] | SFPP [Member] | Pending Litigation [Member]  
Loss Contingencies [Line Items]  
Loss Contingency Period of Time Litigation Concerns 2 years
Annual Rate Reductions [Member] | SFPP [Member] | Pending Litigation [Member]  
SFPP [Abstract]  
Loss Contingency, Damages Sought, Value $ 40
Revenue Subject to Refund [Member] | SFPP [Member] | Pending Litigation [Member]  
SFPP [Abstract]  
Loss Contingency, Damages Sought, Value $ 169