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Commitments and Contingencies - SJGS Matters (Details)	Sep. 30, 2015 MW	Aug. 13, 2015 MW	May. 19, 2015 MW	Jan. 07, 2015 MW	Oct. 01, 2014 MW	Jul. 31, 2014 MW	Jun. 26, 2014 USD ($) MW	Dec. 20, 2013 MW
Clean Air Act, SNCR [Member] \| San Juan Generating Station Unit 4 [Member]
Public Utilities, Commitments And Contingencies [Line Items]
Public utilities, unsubscribed ownership (in megawatts)				65
Public Service Company of New Mexico [Member] \| San Juan Generating Station Unit 4 [Member]
Public Utilities, Commitments And Contingencies [Line Items]
Jointly owned utility plan, proposed proportionate ownership share							64.50%
Public Service Company of New Mexico [Member] \| San Juan Generating Station Units 1 and 4 [Member]
Public Utilities, Commitments And Contingencies [Line Items]
Jointly owned utility plan, proposed proportionate ownership share							58.70%
Public Service Company of New Mexico [Member] \| Clean Air Act, SNCR [Member] \| San Juan Generating Station Unit 3 [Member]
Public Utilities, Commitments And Contingencies [Line Items]
Percentage of ownership held by exiting owners	50.00%
Public utilities, ownership percentage	50.00%
Public Service Company of New Mexico [Member] \| Clean Air Act, SNCR [Member] \| San Juan Generating Station Unit 4 [Member]
Public Utilities, Commitments And Contingencies [Line Items]
Percentage of ownership held by exiting owners	38.80%
Public utilities, ownership percentage	38.50%
Public utilities, additional ownership to be obtained (in megawatts)	132	65	132		132	132	132	78
Public utilities, costs to obtain additional ownership \| $							$ 0
PNMR Development [Member] \| Clean Air Act, SNCR [Member] \| San Juan Generating Station Unit 4 [Member]
Public Utilities, Commitments And Contingencies [Line Items]
Public utilities, potential acquisition of ownership (in megawatts)	65		65