Revenue - Narrative Revenue Term (Details) - Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-10-01	Sep. 30, 2020
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Auction period	3 months
Transmission congestion contracts | Min.
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Auction period	6 months
Transmission congestion contracts | Max.
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Auction period	2 years