Fair Value of Derivative and Other Financial Instruments - Items not Recorded at Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
PNMR
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	$ 2,554,836	$ 2,540,693
Westmoreland Loan	66,588	100,893
Other investments	503	1,164
PNMR | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	2,437,645	2,392,712
Westmoreland Loan	56,640	95,000
Other investments	503	547
PNMR | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	0	0
Westmoreland Loan	0	0
Other investments	503	547
PNMR | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	2,554,836	2,540,693
Westmoreland Loan	0	0
Other investments	0	0
PNMR | Significant Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	0	0
Westmoreland Loan	66,588	100,893
Other investments	0	617
Public Service Company of New Mexico
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	1,727,135	1,730,157
Other investments	283	316
Public Service Company of New Mexico | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	1,657,910	1,631,369
Other investments	283	316
Public Service Company of New Mexico | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	0	0
Other investments	283	316
Public Service Company of New Mexico | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	1,727,135	1,730,157
Other investments	0	0
Public Service Company of New Mexico | Significant Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	0	0
Other investments	0	0
Texas-New Mexico Power Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	527,563	468,329
Other investments	220	231
Texas-New Mexico Power Company | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	480,620	420,875
Other investments	220	231
Texas-New Mexico Power Company | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	0	0
Other investments	220	231
Texas-New Mexico Power Company | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	527,563	468,329
Other investments	0	0
Texas-New Mexico Power Company | Significant Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	0	0
Other investments	$ 0	$ 0