Income Taxes (Details) - USD ($) $ in Millions		3 Months Ended
	May 23, 2017	Mar. 31, 2017	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	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 deferred tax assets as a result of tax rate changes					$ (0.7)
Increase (decrease) in tax expense due to tax rate change (less than for 2017)					0.8
Operating loss carryforward						$ 2.1
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)
Internal Revenue Service (IRS)
Income Tax Contingency [Line Items]
Net refunds received from the IRS				$ 6.5
Increase (decrease) in interest receivable			$ (2.1)
Interest income			5.1
Interest expense			0.7
Professional fees			$ 0.9
Net pre-tax impacts, liability (refund)				(3.5)
Internal Revenue Service (IRS) | Corporate and Other
Income Tax Contingency [Line Items]
Net pre-tax impacts, liability (refund)				(0.6)
Internal Revenue Service (IRS) | PNM
Income Tax Contingency [Line Items]
Net pre-tax impacts, liability (refund)				(2.6)
Internal Revenue Service (IRS) | Texas-New Mexico Power Company
Income Tax Contingency [Line Items]
Net pre-tax impacts, liability (refund)				$ 0.3
NMPRC
Income Tax Contingency [Line Items]
Period of time for proposed return to customers the benefit of the reduction in New Mexico's corporate income tax rate	3 years
PNM
Income Tax Contingency [Line Items]
Increase (decrease) in regulatory liability		(4.8)			$ (7.1)
Increase (decrease) in deferred tax assets as a result of tax rate changes		(0.1)
Increase (decrease) in tax expense due to tax rate change (less than for 2017)		$ 0.1
PNM | NMPRC
Income Tax Contingency [Line Items]
Period of time for proposed return to customers the benefit of the reduction in New Mexico's corporate income tax rate	3 years