Commitments and Contingencies, Purchased Power Agreements (Details) - Independent Power Producing Entities - MW |
9 Months Ended | |
|---|---|---|
Sep. 30, 2017 |
Dec. 31, 2016 |
|
| Purchased Power Agreements [Abstract] | ||
| Generating capacity (in MW) | 897 | 897 |
| Purchase Power Agreement Expiration (year) | 2041 |