Variable Interest Entities (Details) - MW	Sep. 30, 2019	Dec. 31, 2018
Variable Interest Entity [Line Items]
Operating Lease, Weighted Average Remaining Lease Term	8 years 1 month
Purchased Power Agreements [Abstract]
Finance Lease, Weighted Average Remaining Lease Term	38 years 11 months
Independent Power Producing Entities
Purchased Power Agreements [Abstract]
Generating capacity under long term purchased power agreements (in MW)	1,442	1,571