Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2014	Dec. 31, 2013
Income Tax Contingency [Line Items]
New Mexico Corporate tax rate before change in 2013				7.60%
New Mexico Corporate tax rate, effective by 2018			5.90%
Increase (decrease) in regulatory liability	$ (4.8)	$ (7.1)
Corporate and Other
Income Tax Contingency [Line Items]
Increase (decrease) in tax expense due to tax rate change (less than for 2017)	0.1	(0.1)
Public Service Company of New Mexico
Income Tax Contingency [Line Items]
Increase (decrease) in deferred tax assets as a result of tax rate changes	(0.1)	(0.7)
Increase (decrease) in tax expense due to tax rate change (less than for 2017)	$ 0.1	$ 0.8