Regulatory and Rate Matters - Advanced Metering Infrastructure Application and Facebook Data Center Project (Details) $ in Millions		3 Months Ended
	Aug. 24, 2018 Facility power_purchase_agreement MW	Dec. 31, 2017 MW	Dec. 31, 2019 MW	Sep. 05, 2017 MW	Aug. 17, 2016 service_rate	Feb. 26, 2016 USD ($)
Public Utilities, General Disclosures [Line Items]
Number of requested service rates | service_rate					2
Ownership percentage			50.00%
Facebook Data Center
Public Utilities, General Disclosures [Line Items]
PPA term		25 years
Number of additional PPAs | power_purchase_agreement	2
Public Service Company of New Mexico | Advanced metering infrastructure
Public Utilities, General Disclosures [Line Items]
Application to seek recovery of costs of project | $						$ 87.2
Adjustment to estimated costs to be recovered | $						95.1
Application to seek recovery of undepreciated investment | $						$ 33.0
PNMR Development
Public Utilities, General Disclosures [Line Items]
Solar distributed generation (in mw)			80	50
Ownership percentage			50.00%
Casa Mesa Wind, LLC | Facebook Data Center
Public Utilities, General Disclosures [Line Items]
Solar distributed generation (in mw)		50
Avangrid Renewables, LLC | Facebook Data Center
Public Utilities, General Disclosures [Line Items]
Solar distributed generation (in mw)		166
Route 66 Solar Energy Center, LLC | Facebook Data Center
Public Utilities, General Disclosures [Line Items]
Solar distributed generation (in mw)	100	50
Number of solar facilities | Facility	2