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Regulatory and Rate Matters - Formula Transmission Rate Case (Details) - Formula Transmission Rate Case [Member] - Public Service Company of New Mexico [Member] $ in Millions	Mar. 20, 2015 USD ($) party	May. 02, 2014 USD ($)	Jun. 03, 2013 USD ($)	Mar. 01, 2013	Dec. 31, 2012 USD ($)
Public Utilities, General Disclosures [Line Items]
Amount of regulatory costs not yet approved					$ 3.2
Public utilities, return on equity	10.00%				10.81%
Percentage ownership of EIP transmission line				60.00%
Public utilities, requested rate increase (decrease)		$ 0.5	$ 1.3
Number of other parties to settlement | party	5
Public utilities, approved rate increase (decrease)	$ 1.3

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Public Utilities, Return on Equity

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