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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			6 Months Ended
	May. 02, 2014 USD ($)	Jun. 03, 2013 USD ($)	Jun. 30, 2015 USD ($)	Mar. 20, 2015 party
Public Utilities, General Disclosures [Line Items]
Amount of Regulatory Costs Not yet Approved			$ 3.2
Public Utilities, Return on Equity			10.81%	10.00%
Percentage ownership of EIP transmission line			60.00%
Public Utilities, Requested Rate Increase (Decrease), Amount	$ 0.5	$ 1.3
Number of other parties to settlement | party				5
Public Utilities, Approved Rate Increase (Decrease), Amount			$ 1.3

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Amount of regulatory costs not yet approved by the regulatory authority.

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Amount of public utility's requested rate increase (decrease) with regulatory agency.

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