Income Taxes (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended	12 Months Ended
	May 23, 2017	Jun. 30, 2018	Mar. 31, 2017	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2014	Dec. 31, 2013
Income Tax Contingency [Line Items]
New Mexico Corporate tax rate, 2014							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			$ (0.1)
Increase (decrease) in tax expense due to tax rate change (less than for 2017)			0.1
Tax benefits related to stock awards		$ 0.1		$ 1.4
Corporate and Other
Income Tax Contingency [Line Items]
Increase (decrease) in tax expense due to tax rate change (less than for 2017)			0.1
TNMP
Income Tax Contingency [Line Items]
Increase (decrease) in regulatory liability		1.2		2.7
Tax benefits related to stock awards		0.1		0.3
PNM
Income Tax Contingency [Line Items]
Increase (decrease) in regulatory liability			$ (4.8)
Tax benefits related to stock awards		$ 0.1		$ 1.0
Forecast
Income Tax Contingency [Line Items]
Effective tax rate					12.47%
Forecast | TNMP
Income Tax Contingency [Line Items]
Effective tax rate					23.00%
Forecast | PNM
Income Tax Contingency [Line Items]
Effective tax rate					9.08%
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	21 years
NMPRC | PNM
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