Pension and Other Postretirement Benefits - Assumptions Used (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Expected return on plan assets	4.00%
Public Service Company of New Mexico | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Discount rate for determining PBO and APBO	4.05%	4.51%	5.29%
Discount rate for determining net periodic benefit cost (income)	4.51%	5.29%	4.48%
Expected return on plan assets	6.40%	6.50%	6.80%
Public Service Company of New Mexico | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Discount rate for determining PBO and APBO	4.00%	4.47%	5.34%
Discount rate for determining net periodic benefit cost (income)	4.47%	5.34%	4.45%
Expected return on plan assets	7.50%	7.70%	7.70%
Public Service Company of New Mexico | Executive Retirement Program
Defined Benefit Plan Disclosure [Line Items]
Discount rate for determining PBO and APBO	4.05%	4.51%	5.29%
Discount rate for determining net periodic benefit cost (income)	4.51%	5.29%	4.48%
Texas-New Mexico Power Company | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Discount rate for determining PBO and APBO	4.01%	4.49%	5.39%
Discount rate for determining net periodic benefit cost (income)	4.49%	5.39%	4.39%
Expected return on plan assets	6.40%	6.50%	6.80%
Texas-New Mexico Power Company | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Discount rate for determining PBO and APBO	4.00%	4.47%	5.34%
Discount rate for determining net periodic benefit cost (income)	4.47%	5.34%	4.45%
Expected return on plan assets	5.40%	5.70%	5.70%
Texas-New Mexico Power Company | Executive Retirement Program
Defined Benefit Plan Disclosure [Line Items]
Discount rate for determining PBO and APBO	4.01%	4.49%	5.39%
Discount rate for determining net periodic benefit cost (income)	4.49%	5.39%	4.39%