Lease Commitments - Schedule of Weighted Average Remaining Lease Terms and Discount Rates (Details)	Sep. 30, 2020
Weighted average remaining lease term (years):
Operating leases	5 years 11 months 26 days
Financing leases	5 years 25 days
Weighted average discount rate:
Operating leases	3.93%
Financing leases	2.95%
PNM
Weighted average remaining lease term (years):
Operating leases	6 years 2 months 1 day
Financing leases	5 years 25 days
Weighted average discount rate:
Operating leases	3.93%
Financing leases	2.85%
TNMP
Weighted average remaining lease term (years):
Operating leases	3 years 8 months 8 days
Financing leases	5 years 1 month 13 days
Weighted average discount rate:
Operating leases	4.03%
Financing leases	3.05%