Commitments and Contingencies Environmental Requirements - Water and Waste (Details) $ in Millions	Dec. 31, 2019 USD ($) Plant
Federal Coal Ash Regulation [Domain]
Loss Contingencies [Line Items]
Number of regulated ash units that will still be in operation by the end of 2019	3
Number of impoundments where closure plans will be expedited	1
Estimated cost of closure of an impoundment	$ 2.0
Estimated cost of construction of a new impoundment	8.6
Clean Water Act Effluent Limitations Guidelines [Domain]
Loss Contingencies [Line Items]
Liability for estimated cost to comply with regulation	$ 10.0
Federal Clean Water Act Section 316 (b) [Member]
Loss Contingencies [Line Items]
Minimum number of plants which could be required to make improvements to reduce entrainment | Plant	6
Capital Addition Purchase Commitments [Member] | Federal Clean Water Act Section 316 (b) [Member]
Loss Contingencies [Line Items]
Liability for estimated cost to comply with regulation	$ 35.6
Liability for estimated cost to comply with impingement and entrainment regulation	$ 191.6