Commitments, Contingencies And Uncertainties (Narrative) (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended
	Jul. 31, 2011	Sep. 30, 2012	Jan. 19, 2011	Sep. 01, 2009
Number of days to act on a petition			30 days
Number of States Affected By Clean Air Transport Rule	28
Number of criteria emissions established under the Clear Air Act		6
Frequency of Review by Environmental Protection Agency		5 years
Number of facilities required to inventory emissions				10,000
Number of Clean Air Act Programs		2
Accrual for environmental loss contingencies		$ 0.5
Maximum
Mercury and Air Toxics Standards Required Compliance Period, in Years		5 years
Defined increase in GHG emissions per year, in tons		100,000
Minimum
Mercury and Air Toxics Standards Required Compliance Period, in Years		4 years
Defined increase in GHG emissions per year, in tons		75,000
Four Corners Generating Station
Accrual for environmental loss contingencies		0.2
Four Corners Generating Station | Maximum
Loss Contingency, Estimate of Possible Loss		0.9
Four Corners Generating Station | Minimum
Loss Contingency, Estimate of Possible Loss		$ 0.2