Commitments and Contingencies - National Ambient Air Quality Standards (Details) - Public Service Company of New Mexico	Feb. 25, 2016	Mar. 02, 2015 lb / MMBTU T	Oct. 01, 2015 parts_per_billion	Sep. 30, 2015 parts_per_billion	May 14, 2015 lb / MMBTU
Maximum | San Juan Generating Station And Four Corners
Public Utilities, General Disclosures [Line Items]
Government standard emissions limit (in parts per billion) | parts_per_billion			70	75
National Ambient Air Quality Standards, 2015 EPA Legal Settlement
Public Utilities, General Disclosures [Line Items]
Period of time to act on settlement		16 months
Emissions tons of SO2 per year (more than) | T		16,000
National Ambient Air Quality Standards, 2015 EPA Legal Settlement | Minimum
Public Utilities, General Disclosures [Line Items]
Emissions tons of SO2 per year (more than) | T		2,600
SO2 emissions rate (in pounds per MMBTU) | lb / MMBTU		0.45
National Ambient Air Quality Standards | San Juan Generating Station
Public Utilities, General Disclosures [Line Items]
Revised SO2 emissions agreed upon (in pounds per MMBTU) | lb / MMBTU					0.10
National Ambient Air Quality Standards, Proposed 2016 SO2 Rule
Public Utilities, General Disclosures [Line Items]
Period of time from state designation to provide implementation plans	36 months