Income Taxes (Details) - USD ($)		3 Months Ended	9 Months Ended	12 Months Ended
	Jan. 10, 2018	Sep. 30, 2020	Sep. 30, 2020	Dec. 31, 2017
Income Tax Contingency [Line Items]
Accelerated refund application amount of AMT tax, CARES act		$ 5,200,000	$ 5,200,000
Deferred federal, state and local, income tax expense (benefit)			$ 30,700,000
Effective tax rate			8.44%
Tax benefits related to stock awards		0	$ 400,000
PNM
Income Tax Contingency [Line Items]
Deferred federal, state and local, income tax expense (benefit)			$ 21,600,000
Effective tax rate			11.29%
Tax benefits related to stock awards		0	$ 300,000
TNMP
Income Tax Contingency [Line Items]
Deferred federal, state and local, income tax expense (benefit)			$ 9,100,000
Effective tax rate			8.83%
Tax benefits related to stock awards		$ 0	$ 100,000
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