Commitments and Contingencies, Environmental Contingencies - Unrecorded Unconditional Purchase Obligation (Details) $ in Millions |
9 Months Ended |
|---|---|
|
Sep. 30, 2017
USD ($)
| |
| Clean Power Plan [Member] | |
| Unrecorded Unconditional Purchase Obligation [Line Items] | |
| Length of Public Comment Period | 60 days |
| Regional Haze Rules | |
| Environmental Requirements [Abstract] | |
| Number of units receiving allocated allowances under program | 3 |
| Harrington Units 1 and 2 [Member] | Capital Addition Purchase Commitments | Regional Haze Rules | |
| Environmental Requirements [Abstract] | |
| Liability for estimated cost to comply with regulation | $ 400 |
| Tolk Units 1 and 2 [Member] | Capital Addition Purchase Commitments | Regional Haze Rules | |
| Environmental Requirements [Abstract] | |
| Liability for estimated cost to comply with regulation | $ 600 |