Pension and Other Postretirement Benefits - Other Postretirement Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
401(k) plan
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Abstract]
401(k) plan	$ 16,247	$ 16,097	$ 16,677
401(k) plan | Public Service Company of New Mexico
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Abstract]
401(k) plan	11,676	11,587	12,052
401(k) plan | Texas-New Mexico Power Company
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Abstract]
401(k) plan	4,572	4,511	4,625
Non-qualified plan
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Abstract]
Non-qualified plan	2,090	4,551	865
Non-qualified plan | Public Service Company of New Mexico
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Abstract]
Non-qualified plan	1,544	3,384	621
Non-qualified plan | Texas-New Mexico Power Company
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Abstract]
Non-qualified plan	$ 547	$ 1,167	$ 244