Commitments, Contingencies and Uncertainties (Narrative) (Details) $ in Millions		12 Months Ended
	Aug. 17, 2015 USD ($)	Dec. 31, 2015 USD ($) Decimals	Jul. 28, 2015 States
Loss Contingencies [Line Items]
Number Of States Affected By EPA's Emissions Budgets | States			13
Number of criteria emissions established under the Clear Air Act		6
Frequency of Review by Environmental Protection Agency		5 years
Jointly Owned Utility Plant, Proportionate Ownership Share		7.00%
Accrual for Environmental Loss Contingencies		$ 0.5
Four Corners Coal Plant Participants
Loss Contingencies [Line Items]
Civil Penalty Amount	$ 1.5
Environmental Mitigation Project Cost	$ 6.7
Public Utilities, Assessed Coal Severance Surtax Penalty and Interest		$ 30.0
Four Corners Generating Station
Loss Contingencies [Line Items]
Date participation is ceased		Jul. 06, 2016
Public Utilities, Assessed Coal Severance Surtax Penalty and Interest		$ 1.5
Minimum
Loss Contingencies [Line Items]
Public Utilities, Government Standard Emission Limit | Decimals		70
Maximum
Loss Contingencies [Line Items]
Public Utilities, Government Standard Emission Limit | Decimals		75