Commitments and Contingencies, Environmental Contingencies (Details) (USD $) In Millions, unless otherwise specified			1 Months Ended	6 Months Ended
	Jun. 30, 2012 New Mexico Greenhouse Gas (GHG) Regulations [Member]	Mar. 31, 2012 New Mexico Greenhouse Gas (GHG) Regulations [Member]	Apr. 30, 2012 Electric Generating Unit (EGU) Mercury and Air Toxics Standards (MATS) Rule [Member] MW	Jun. 30, 2012 Capital Commitments [Member] Cross-State Air Pollution Rule (CSAPR) [Member]
Environmental Requirements [Abstract]
Number of regulations in New Mexico to limit GHG emissions which were repealed in 2012		2
Number of appeals in which intervention has been granted	1
Number of petitions filed to intervene in the second appeal	1
Liability for estimated cost to comply with regulation				$ 470
Number of years in which capital expenditures are expected to be incurred (in years)				5Y
Generating capacity (in MW)			25
Number of years before affected facilities must demonstrate compliance, low end of range (in years)			3Y
Number of years before affected facilities must demonstrate compliance, high end of range (in years)			4Y