Income Taxes - Reconciliation of Unrecognized Tax Benefits (Expense) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits, interest on income taxes expense	$ 0	$ 0	$ 0
PNMR
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Beginning balance	10,194,000	9,429,000	6,752,000
Additions based on tax positions	329,000	543,000	262,000
Additions (reductions) for tax positions of prior years	170,000	222,000	2,415,000
Settlement payments	0	0	0
Ending balance	10,693,000	10,194,000	9,429,000
Unrecognized tax benefits that would impact effective tax rate	10,100,000
Public Service Company of New Mexico
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Beginning balance	7,288,000	6,563,000	3,949,000
Additions based on tax positions	329,000	543,000	262,000
Additions (reductions) for tax positions of prior years	159,000	182,000	2,352,000
Settlement payments	0	0	0
Ending balance	7,776,000	7,288,000	6,563,000
Unrecognized tax benefits that would impact effective tax rate	7,200,000
Unrecognized tax benefits, interest on income taxes expense	0	0	0
Texas-New Mexico Power Company
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Beginning balance	103,000	63,000	0
Additions based on tax positions	0	0	0
Additions (reductions) for tax positions of prior years	11,000	40,000	63,000
Settlement payments	0	0	0
Ending balance	114,000	103,000	63,000
Unrecognized tax benefits that would impact effective tax rate	100,000
Unrecognized tax benefits, interest on income taxes expense	$ 0	$ 0	$ 0