Regulatory and Rate Matters - Firm-Requirements Wholesale Customers (Details) - Public Service Company of New Mexico [Member] $ in Millions		3 Months Ended	6 Months Ended	12 Months Ended
	Dec. 06, 2012 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Dec. 31, 2017 MW	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Firm Requirements Wholesale Power Rate Case, Navopache [Member] [Member]
Public Utilities, General Disclosures [Line Items]
Approved rate increase (decrease)	$ 5.3
Revenue for power sold under specific contract					$ 20.0	$ 27.1	$ 28.4
Firm Requirements Wholesale Power Rate Case, Navopache [Member] [Member] | Scenario, Forecast [Member]
Public Utilities, General Disclosures [Line Items]
Serving megawatts of load under a lower tariff (in mw) | MW				10
Firm Requirements Wholesale Power Rate Case [Member]
Public Utilities, General Disclosures [Line Items]
Contract extension	10 years
Firm Requirements Wholesale Power Contract, Gallup [Member]
Public Utilities, General Disclosures [Line Items]
Revenue for power sold under specific contract			$ 6.1
City of Gallup, New Mexico Contract [Member]
Public Utilities, General Disclosures [Line Items]
Gain on sale of substation		$ 1.1