Regulatory and Rate Matters - Formula Transmission Rate Case (Details) - Public Service Company of New Mexico [Member] $ in Millions			12 Months Ended
	Mar. 20, 2015 USD ($) party	Mar. 01, 2013	Dec. 31, 2012 USD ($)
Wholesale Electric Transmission Rate Case [Member] [Member]
Public Utilities, General Disclosures [Line Items]
Approved rate increase (decrease)			$ 3.2
Initial approved return on equity			10.81%
Lease ownership in EIP		60.00%
Approved additional rate increase	$ 1.3
Formula Transmission Rate Case [Member]
Public Utilities, General Disclosures [Line Items]
Number of additional parties entered into settlement agreement (in parties) | party	5
Return on equity	10.00%