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Commitments and Contingencies - National Ambient Air Quality Standards (Details) - Public Service Company of New Mexico [Member]	Mar. 02, 2015 lb / MMBTU T	Oct. 01, 2015 opp	Sep. 30, 2015 opp	Dec. 17, 2014 opp	Nov. 25, 2014 opp	Nov. 08, 2013 lb / MMBTU	Jan. 31, 2010 opp
National Ambient Air Quality Standards, 2015 EPA Legal Settlement [Member]
Public Utilities, Commitments And Contingencies [Line Items]
Period of time to act on settlement	16 months
Public utilities, emissions of SO2 per year (in tons) \| T	16,000
San Juan Generating Station [Member] \| National Ambient Air Quality Standards [Member]
Public Utilities, Commitments And Contingencies [Line Items]
Public utilities, revised SO2 emissions, agreed upon (in pounds per MMBTU) \| lb / MMBTU						0.10
Minimum [Member] \| National Ambient Air Quality Standards, 2015 EPA Legal Settlement [Member]
Public Utilities, Commitments And Contingencies [Line Items]
Public utilities, emissions of SO2 per year (in tons) \| T	2,600
Public utilities, 1-hour SO2 emissions rate (in pounds per MMBTU) \| lb / MMBTU	0.45
Minimum [Member] \| San Juan Generating Station And Four Corners [Member]
Public Utilities, Commitments And Contingencies [Line Items]
Public utilities, proposed government standard emission limit (in ozone parts per million)				65	65		60
Maximum [Member] \| San Juan Generating Station And Four Corners [Member]
Public Utilities, Commitments And Contingencies [Line Items]
Public utilities, proposed government standard emission limit (in ozone parts per million)				70			70
Public utilities, government standard emission limit (in ozone parts per million)			75
Subsequent Event [Member] \| Maximum [Member] \| San Juan Generating Station And Four Corners [Member]
Public Utilities, Commitments And Contingencies [Line Items]
Public utilities, government standard emission limit (in ozone parts per million)		70