Commitments and Contingencies - Coal Supply (Details)			3 Months Ended		9 Months Ended		12 Months Ended
	Feb. 01, 2016 USD ($)	Jan. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Dec. 31, 2015 USD ($)	Nov. 01, 2016 USD ($)	Oct. 21, 2016 USD ($)	Aug. 31, 2016 USD ($)	Aug. 30, 2016 USD ($)	Aug. 01, 2016 USD ($) payment	Jul. 19, 2016 USD ($)	Apr. 30, 2016 USD ($)
Public Utilities, Commitments And Contingencies [Line Items]
Other current assets			$ 89,746,000		$ 89,746,000		$ 82,104,000
Regulatory disallowances and restructuring costs			16,451,000	$ 0	17,225,000	$ 1,744,000
Mine reclamation trust requirement funded			1,900,000
San Juan Generating Station [Member] | Coal Supply [Member]
Public Utilities, Commitments And Contingencies [Line Items]
Replacement Coal Mine Reclamation Bonds posted by alternate surety companies													$ 118,700,000
Public Service Company of New Mexico [Member]
Public Utilities, Commitments And Contingencies [Line Items]
Other current assets			81,419,000		81,419,000		74,990,000
Regulatory disallowances and restructuring costs			16,451,000	$ 0	17,225,000	$ 1,744,000
Public Service Company of New Mexico [Member] | Increase in coal mine decommissioning liability [Member]
Public Utilities, Commitments And Contingencies [Line Items]
Expense reflected in regulatory disallowances and restructuring costs			4,800,000
Public Service Company of New Mexico [Member] | Mine Reclamation Trust [Member]
Public Utilities, Commitments And Contingencies [Line Items]
Required contribution to Reclamation Trust, current fiscal year			3,500,000		3,500,000
Anticipated funding to Reclamation Trust, next fiscal year			4,700,000		4,700,000
Anticipated funding to Reclamation Trust, third fiscal year			5,100,000		5,100,000
Public Service Company of New Mexico [Member] | Mine Reclamation Trust [Member] | Four Corners [Member]
Public Utilities, Commitments And Contingencies [Line Items]
Anticipated funding to Reclamation Trust, next fiscal year			2,000,000		2,000,000
Anticipated funding to Reclamation Trust, third fiscal year			2,000,000		2,000,000
Number of annual installment payments | payment												13
Public Service Company of New Mexico [Member] | Surface [Member]
Public Utilities, Commitments And Contingencies [Line Items]
Regulatory assets			100,000,000		100,000,000
Public Service Company of New Mexico [Member] | Loss on long-term purchase commitment [Member] | Surface [Member]
Public Utilities, Commitments And Contingencies [Line Items]
Loss contingency accrual			41,400,000		41,400,000		38,800,000
Final reclamation, capped amount to be collected					100,000,000.0
Public Service Company of New Mexico [Member] | Loss on long-term purchase commitment [Member] | Underground [Member]
Public Utilities, Commitments And Contingencies [Line Items]
Loss contingency accrual			13,900,000		13,900,000		11,400,000
Public Service Company of New Mexico [Member] | San Juan Generating Station [Member] | Loss on long-term purchase commitment [Member] | Surface [Member]
Public Utilities, Commitments And Contingencies [Line Items]
Regulatory disallowances and restructuring costs							16,500,000
Estimate of possible loss			103,000,000		103,000,000
Public Service Company of New Mexico [Member] | San Juan Generating Station [Member] | Loss on long-term purchase commitment [Member] | Underground [Member]
Public Utilities, Commitments And Contingencies [Line Items]
Estimate of possible loss			129,400,000		129,400,000
Public Service Company of New Mexico [Member] | San Juan Generating Station [Member] | Coal Supply [Member]
Public Utilities, Commitments And Contingencies [Line Items]
Other current assets			$ 54,100,000		$ 54,100,000		49,000,000
Estimated increase in coal cost			40.00%		40.00%
NM Capital [Member] | BTMU Term Loan Agreement [Member]
Public Utilities, Commitments And Contingencies [Line Items]
Term loans	$ 125,000,000.0		$ 107,800,000.0		$ 107,800,000.0
NM Capital [Member] | BTMU Term Loan Agreement [Member] | Maximum [Member]
Public Utilities, Commitments And Contingencies [Line Items]
Debt-to-capital ratio covenant	65.00%
NM Capital [Member] | San Juan Generating Station [Member] | Coal Supply [Member]
Public Utilities, Commitments And Contingencies [Line Items]
Payments to fund long-term loans to unaffiliated third party		$ 125,000,000.0
Loan agreement among several entities		$ 125,000,000
Initial interest rate		7.25%
Requirement to post reclamation bonds							161,600,000							$ 118,700,000
Cash used to support bank letter or credit arrangement							$ 40,000,000						$ 30,300,000
NM Capital [Member] | San Juan Coal Company, Westmoreland [Member] | Coal Supply [Member]
Public Utilities, Commitments And Contingencies [Line Items]
Payments to fund long-term loans to unaffiliated third party	$ 125,000,000.0
Loan receivable		$ 125,000,000	110,000,000		110,000,000
Scheduled principal payment proceeds												$ 15,000,000
Prepayment penalty			$ 0		$ 0
Cash used to support bank letter or credit arrangement										$ 30,300,000	$ 40,000,000
NM Capital [Member] | San Juan Coal Company, Westmoreland [Member] | Coal Supply [Member] | Subsequent event [Member]
Public Utilities, Commitments And Contingencies [Line Items]
Scheduled principal payment proceeds								$ 15,000,000
Scheduled interest payment proceeds								$ 2,300,000
Unaffiliated third party [Member] | San Juan Coal Company, Westmoreland [Member] | Coal Supply [Member] | Subsequent event [Member]
Public Utilities, Commitments And Contingencies [Line Items]
Amount held in a restricted bank account									$ 17,300,000