Income Taxes (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended	6 Months Ended	12 Months Ended
	Jun. 30, 2021	Jun. 30, 2021	Jun. 30, 2021	Dec. 31, 2017	Dec. 31, 2020
Income Tax Contingency [Line Items]
Accelerated refund application amount of AMT tax, CARES act					$ 5.2
Deferred tax assets, tax deferred expense, payroll taxes, CARES Act					$ 7.0
Refund amount of AMT tax, CARES act	$ 5.2
Deferred federal, state and local, income tax expense (benefit)			$ 24.5
Effective tax rate			13.35%
Tax benefits related to stock awards		$ 0.1	$ 0.8
Tax benefits on merger-related costs		0.3	1.6
PNM
Income Tax Contingency [Line Items]
Deferred federal, state and local, income tax expense (benefit)			$ 15.2
Effective tax rate			15.81%
Tax benefits related to stock awards		0.1	$ 0.6
Tax benefits on merger-related costs		0.1	0.1
TNMP
Income Tax Contingency [Line Items]
Deferred federal, state and local, income tax expense (benefit)			$ 9.3
Effective tax rate			10.64%
Tax benefits related to stock awards		0.1	$ 0.2
Tax benefits on merger-related costs		$ 0.1	$ 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