Rate Matters (Details) (USD $) In Millions, unless otherwise specified	0 Months Ended	1 Months Ended
	Mar. 29, 2013 Certain Texas Transmission Assets [Member] Facility	Nov. 30, 2012 Public Utility Commission of Texas (PUCT) [Member] 2012 Electric Rate Case [Member]	Apr. 30, 2013 Public Utility Commission of Texas (PUCT) [Member] 2012 Electric Rate Case, Settlement Rates Effective May 1, 2013 [Member]	Apr. 30, 2013 Public Utility Commission of Texas (PUCT) [Member] 2012 Electric Rate Case, Settlement Rates Effective September 1, 2013 [Member]	Mar. 31, 2013 New Mexico Public Regulation Commission (NMPRC) [Member] 2012 Electric Rate Case [Member]	Dec. 31, 2012 New Mexico Public Regulation Commission (NMPRC) [Member] 2012 Electric Rate Case [Member]
Rate Matters [Abstract]
Entity's requested rate increase (decrease)		$ 90.2				$ 45.9
Number of months included in test year for rate filing (in months)		12 months
Entity's requested return on equity (in hundredths)		10.65%				10.65%
Entity's rate base in requested rate increase (decrease)		1,150.0				479.8
Entity's requested equity ratio percent (in hundredths)		52.00%				53.89%
Entity's revised requested rate increase (decrease)			37.0	13.8
Term of initial suspension period for tariffs					9 months
Term of additional suspension period for tariffs					3 months
Number of substations included in purchase and sale agreement	2
Base purchase price in purchase and sale agreement	$ 37