Commitments and Contingencies - Coal Supply (Details)					12 Months Ended
	Feb. 01, 2018 USD ($)	Feb. 01, 2016 USD ($)	Jan. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Feb. 20, 2018 USD ($)	Jun. 30, 2017 USD ($)	Feb. 01, 2017	Jul. 19, 2016 USD ($)	Jul. 07, 2016 payment	Mar. 31, 2016 USD ($)
Other Commitments [Line Items]
Other current assets					$ 47,358,000	$ 73,444,000
Regulatory disallowances and restructuring costs					27,036,000	15,011,000	$ 167,471,000
Funds contributed to mine reclamation trust					2,300,000	1,900,000
Public Service Company of New Mexico
Other Commitments [Line Items]
Other current assets					39,904,000	67,355,000
Regulatory disallowances and restructuring costs					27,036,000	15,011,000	167,471,000
Regulatory assets					461,378,000	368,855,000
Public Service Company of New Mexico | Increase in coal mine decommissioning liability
Other Commitments [Line Items]
Net expense					4,500,000
Public Service Company of New Mexico | Loss on long-term purchase commitment | San Juan Generating Station
Other Commitments [Line Items]
Annual funding post-term reclamation trust					5,800,000	7,000,000	4,300,000
Public Service Company of New Mexico | Mine Reclamation Trust | San Juan Generating Station
Other Commitments [Line Items]
Reclamation trust funding, next fiscal year					8,300,000
Reclamation trust funding, year 2					8,700,000
Reclamation trust funding, year 3					9,200,000
Public Service Company of New Mexico | Mine Reclamation Trust | Four Corners
Other Commitments [Line Items]
Reclamation trust funding, next fiscal year					2,300,000
Reclamation trust funding, year 2					2,300,000
Reclamation trust funding, year 3					2,300,000
Number of annual installments (in payments) | payment												13
Public Service Company of New Mexico | Surface
Other Commitments [Line Items]
Regulatory assets					100,000,000
Public Service Company of New Mexico | Surface | Loss on long-term purchase commitment
Other Commitments [Line Items]
Liability for interim storage costs					41,400,000	41,000,000
Increase (decrease) for interim storage costs liability					35,000,000
Capped amount that can be collected from retail customers for final reclamation					100,000,000.0
Public Service Company of New Mexico | Underground | Loss on long-term purchase commitment
Other Commitments [Line Items]
Liability for interim storage costs					14,700,000	14,000,000
Increase (decrease) for interim storage costs liability					5,000,000
Public Service Company of New Mexico | San Juan Generating Station | Surface | Loss on long-term purchase commitment
Other Commitments [Line Items]
Regulatory disallowances and restructuring costs				$ 16,500,000			$ 16,500,000
Estimate of possible loss					100,400,000
Public Service Company of New Mexico | San Juan Generating Station | Underground | Loss on long-term purchase commitment
Other Commitments [Line Items]
Estimate of possible loss					127,100,000
Public Service Company of New Mexico | San Juan Generating Station | Coal Supply
Other Commitments [Line Items]
Other current assets					26,300,000	$ 48,700,000
Estimated increase in coal cost									51.00%
Public Service Company of New Mexico | Four Corners
Other Commitments [Line Items]
Unrecorded unconditional purchase obligation									$ 6,500,000
NM Capital | BTMU Term Loan Agreement
Other Commitments [Line Items]
Long-term debt		$ 125,000,000.0			50,100,000
NM Capital | BTMU Term Loan Agreement | Subsequent Event
Other Commitments [Line Items]
Long-term debt								$ 45,100,000
NM Capital | San Juan Generating Station | Coal Supply
Other Commitments [Line Items]
Funding provided		$ 125,000,000.0	$ 125,000,000.0
Variable interest rate		7.25%								9.25%
Coal mine reclamation bonds to be posted with NMMMD													$ 118,700,000
Cash used to support bank letter or credit arrangement			30,300,000								$ 30,300,000
NM Capital | San Juan Generating Station | Coal Supply | Subsequent Event
Other Commitments [Line Items]
Variable interest rate	12.25%
NM Capital | San Juan Generating Station | Coal Supply | Subsequent Event | Westmoreland San Juan
Other Commitments [Line Items]
Principal payment received	$ 5,600,000
NM Capital | San Juan Coal Company, Westmoreland | Coal Supply
Other Commitments [Line Items]
Amount outstanding			$ 125,000,000		56,600,000
Prepayment penalty					$ 0
NM Capital | San Juan Coal Company, Westmoreland | Coal Supply | Subsequent Event
Other Commitments [Line Items]
Amount outstanding								$ 51,000,000