Income Taxes (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Income Tax Contingency [Line Items]
New Mexico Corporate tax rate, current	7.60%
New Mexico Corporate tax rate, 2014	5.90%
Decrease in regulatory liabilities due to change in state corporate tax rate	$ 2.0	$ 4.6
Decrease in deferred tax asset due to change in state corporate tax rate		0.2
State Net Operating Loss Carryforward, Impairment	1.0
Public Service Company of New Mexico [Member]
Income Tax Contingency [Line Items]
Increase in deferred tax asset not related to regulatory activity, as a result of tax rate change	0.7
Decrease in income tax expense due to tax rate change	0.5
State Net Operating Loss Carryforward, Impairment	0.7
Corporate and Other [Member]
Income Tax Contingency [Line Items]
Decrease in income tax expense due to tax rate change	0.2
State Net Operating Loss Carryforward, Impairment	$ 0.3