Income Taxes (Details) - USD ($) $ in Millions		3 Months Ended	12 Months Ended
	Jan. 10, 2018	Mar. 31, 2020	Dec. 31, 2017
Income Tax Contingency [Line Items]
Deferred federal, state and local, income tax expense (benefit)		$ 30.3
Effective tax rate		8.63%
Tax benefits related to stock awards		$ 0.4
PNM
Income Tax Contingency [Line Items]
Deferred federal, state and local, income tax expense (benefit)		$ 21.2
Effective tax rate		11.28%
Tax benefits related to stock awards		$ 0.3
TNMP
Income Tax Contingency [Line Items]
Deferred federal, state and local, income tax expense (benefit)		$ 9.1
Effective tax rate		9.79%
Tax benefits related to stock awards		$ 0.1
NMPRC
Income Tax Contingency [Line Items]
Period of time for proposed return to customers the benefit of the reduction in federal corporate income tax rate			23 years
Period of time for proposed return to customers the benefit of the reduction in New Mexico's corporate income tax rate			3 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