Commitments and Contingencies - Right-of-Way, Complaints, and Navajo National Allottee Matters (Details) $ in Millions			3 Months Ended
	Jan. 04, 2016	Oct. 29, 2015 USD ($)	Dec. 31, 2017 USD ($)	Nov. 20, 2017 Allotment_Parcel	Dec. 01, 2015 Allotment_Parcel	Jul. 13, 2015 a	Jan. 22, 2015 Allotment_Parcel	Apr. 02, 2014 landowner Allotment_Parcel	Feb. 27, 2014 lawsuit	Jan. 06, 2014 Allotment_Parcel	Sep. 30, 2012 landowner
First Choice
Loss Contingencies [Line Items]
Additional tax due plus penalties and interest | $			$ 5.0
SPS Complaint, Federal Power Act | Public Service Company of New Mexico
Loss Contingencies [Line Items]
Amount received, including interest, under settlement | $		$ 4.2
Settlement amount passed back to customers | $		$ 2.6
Navajo Nation Allottee Matters | Public Service Company of New Mexico
Loss Contingencies [Line Items]
Number of landowners claiming to be Navajo allottees (in landowners) | landowner								43			43
Number of appraisals requested for review (in allotment parcels)										58
Number of allotments where landowners are revoking rights of way renewal consents (in allotment parcels)								6
Allotments with right-of-way renewals not previously contested (in allotment parcels)							10
Acres of land at issue (in acres) | a						15.49
Number of allotment parcels that cannot be condemned				2	2
Number of allotment parcels at issue					5
Pending Litigation
Loss Contingencies [Line Items]
Number of claims (in lawsuits) | lawsuit									2
Written notification requirement to terminate agreement, minimum period	30 days
Settled Litigation
Loss Contingencies [Line Items]
Number of claims (in lawsuits) | lawsuit									1