Commitments, Contingencies and Uncertainties (Narrative) (Details) $ in Millions	Aug. 17, 2015 USD ($)	Dec. 31, 2018 USD ($)
Loss Contingencies [Line Items]
Number of criteria emissions established under the Clear Air Act		6
Accrual for Environmental Loss Contingencies		$ 0.2
Four Corners Coal Plant Participants
Loss Contingencies [Line Items]
Civil Penalty Amount	$ 1.5
Environmental Mitigation Project Cost	$ 6.7
Maximum
Loss Contingencies [Line Items]
Public Utilities, Government Standard Emission Limit		75
Minimum
Loss Contingencies [Line Items]
Public Utilities, Government Standard Emission Limit		70