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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 |  |

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