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